In the Volla Forum a solution is discussed how to get back from Volla 0S12 to Volla OS 11.
CAUTION: All user data will be lost!
Detais:
https://forum.volla.online/viewtopic.php?f=31&t=1042&p=4616&hilit=flash#p4616
Quick guide Flashing with the SP Flashtool
from: waldbursche Thu 14 Dec 2023, 23:55
I Used the Solution several times.

@Idcrafter
Multi Boot runs very well on a Volla Phone 22 - if the VOLLA OS 11.1 version of 17.10.2022 is used (was delivered with this version in October 2023).
And yes there are updates to newer versions (Volla OS 11.1 from 11.01.2023 and 16.06.2023). But I am not sure if these versions are suitable for Multi Boot. AND CAUTION: With Volla OS 12.x no Multi Boot is currently possible. Volla is working on it. For perspective see https://volla.online/en/blog/files/november-update-23.html.
The additional operating systems are stored on an SD card with an (automatically generated) boot manager. The SD cards can be changed (and therefore also copied) without any disadvantages. Volla OS accesses the boot manager of the SD card and starts the additional operating systems from the SD card.
The installation of Volla OS 11.1 is possible with UBports. Stand-alone installations such as Ubuntu Touch are overwritten. When installing Volla OS with UBports, all initializations and user data will be lost!

On a Volla Phone 22 MultiBoot does not run if Volla OS 12 is installed. See also https://volla.online/en/blog/files/august-updtte-23.html (...Volla OS 12 ...an update of the hardware abstraction layer for the Volla Phone 22 is required due to new device drivers ...).
With Volla OS 11 I have good experience with MultiBoot.

Helpful information about MultiBoot on a Volla Phone 22 is available in the Volla Blog https://volla.online/en/blog/ (August Update 13/08/23) and in the Volla Help https://volla.online/en/faqs/ (How can I install another operating system?).
If on the Volla Phone "only" Ubuntu Touch is installed: Volla OS 11is available in UBports installer 0.10.0.
But: on the Volla Phone 22 MultiBoot can currently only be realized with Volla OS 11! With Volla OS 12 no MultiBoot can be realized. A Volla Phone 22 running Volla OS 12 lists additional operating systems, but cannot boot them. See also Volla Blog https://volla.online/en/blog/ (August Update 13/08/23): "... an update of the hardware abstraction layer is required for the Volla Phone 22 due to new device drivers ..."
How to get from Volla OS 12 back to Volla OS 11 is unclear to me. My attempts resulted in the Volla Phone 22 not booting anything after installing Volla OS (11) with UBports when Volla OS 12 was already installed! So be carefull.

@Marathon2422
I've tried the adaptation from the video several times. I had no luck with it.

@ezze
Of course, we have to be careful when installing files that come from unknown sites. And: It seems to me that all devicesget the same MAC. That does not bother me. I do not do banking with the tablet, I do not mind with google or any "social networks". I would not even look at my e-mails with a tablet.
Anyway, when I flash cool-6, all the modifications are gone. I get back the files "WIFI" and "WIFI_CUSTOM" that do not run.
But:
Maybe someone can work up the facts to the extent that CONTROLLED and SAFE files "WIFI" and "WIFI_CUSTOM" are delivered during the installation process of the ubports-installer. Unfortunately I am not able to do that.
It seems that not only BQ M10 tablets are affected.

How to get rid of the following error message when entering the command line
'sudo apt-get update':
'W: No sandbox user' _apt 'on the system, can not drop privileges'
Is the message a problem at all?

@1896er
The problem is solvable. Please refer to
https://forum.xda-developers.com/showthread.php?t=2730486
At the bottom of the page you will find the text:
Attached Files
Wifi Fix Files.rar - [Click for QR Code] (187 Bytes, 339712 views)
After downloading and unzipping the file Wifi Fix Files.rar you get two files:
WIFI and WIFI_CUSTOM
These two files must be in the folder
/Data/nvram/APCFG/APRDEB
be copied (overwrite the existing files)
In conclusion, do not forget reboot.

Unfortunately my statement of 11 Oct 2019, 16:53 is not the whole truth. LibreOffice and Firefox are running sometimes on libertine (but sometimes not). Unfortunately, it does not always work when I create a new container.
So if necessary, remove the existing container and build a new one. Maybe the following PPAs will help as well:
ppa: ubports-developers / overlay
ppa: ubuntu-touch-core apps-drivers / collection
ppa: libreoffice / ppa
ppa: mozilla team / firefox-next

@Marathon2422
I had done the "repairing by copying 'WIFI' and 'WIFI-CuSTOM' into the path '/data / nvram / APCFG / APRDEB /' " on a fresh installation of "devil" (on 2019-10-20). After changing to "rc" (also 2019-10-20) the Wifi address was unchanged and Wifi was automatically connected.
The problem seems not to be Ubports, but the basic flashing of "cooler" (for BQ_M10_HD) with "SP_Flash_Tool_v5.1824_Linux". I already tested that a few days ago.
Flashing again UBports installer (0.2.7-beta) (at 2019-10-20 to the installation from 2019-10-19) did not change the wifi address. So I had not to repair the WIFI again. My wifi network connected automatically immediately.
Wifi_fix will only be necessary if you switch (comming from Android or original BQ-Ubuntu via) "cooler" to Ubports.

After months of searching, I finally found a solution for the problem - that even I understood.
With some commands in the terminal, the error can be corrected. Please refer to:
https://forum.xda-developers.com/showthread.php?t=2730486
Thanks to Nilendu from India

Using Libertine Firefox 69.0.2 can be used with Libertine.
Using Libertine: LibreOffice can be installed but does not run.
This is true for BQ M10 HD - Ubports 16.04/stable 2019-10-08 and
Fairphone2 - Ubports 16.04/devel 2019-10-11.

@emailforfor At least I tested "Import" / "EXPORT" of bookmarks and unfortunately there is a mistake in my information from 30.09.2019! The bookmarks are stored in "/home/phablet/.local/share/morph-browser/bookmarks.sqlite". Sorry abaut that.

Today there is an easier way to "EXPORT" an "IMPORT" the telephone-book (and the bookmarks).
With FILE MANAGER the file
"/home/phablet/.local/share/evolution/addressbook/system/contacts.db"
can be copied. To install the file on another UT device, first delete the existing file at this path and then copy the database to be imported into the path described above.
I did not test import/export of bookmarks. With the morph-browser you'll find 2 databases in path
"/home/phablet/.local/share/morph-browser/databases/":
"Databases.db" and "Databases.db-jounal"
Remember: reboot is required.

To bring it to an end.
I contacted BQ several times and pointed out the problems:

1. M10 does not restore the wireless connection after a reboot.
2. the MAC address is changed after each restart.
   BQ has always pointed to the possibility of reinstalling the firmware (see the homepage of BQ). Several times I have reported to BQ that I have done exactly that but the error was not corrected. I also asked for a repair the instrument.
   BQ did not respond to my questions at all. Either BQ is not able to offer a solution, or does not worry about the Ubuntu devices (BQ does not offer devices with Linux any longer).
   I think: The BQ service does not understand the problem or is aware of it and tries to get rid of this questioner. The only alternative to repair the M10 was an offer about M10-HD with OTA-9.5, 20160215,018653 (150 €). To install Ubuntu Touch on such an instrument you first have to flash the (much newer firmware) cooler-image-stable-6. ATTENTION! IF YOU LATER CHANGE TO ANDROID, WIFI IS CONNECTED AFTER RESTART. IF YOU THEN TRY TO RETURN TO COOLER-IMAGE-STABLE-6 or UBUNTU TOUCH, WIFI WILL NEVER CONNECTED AFTER A RESTART AGAIN IF YOU DO NOT USE ANDROID! But who wants to use Android, if he has bought the M10 Ubuntu Edition.
   My advice: If a BQ M10 does not connect to Wi-Fi after a reboot: Forget the BQ service! Do not shut down the device / WLAN. Simply switch to stand by mode. The accu of a M10 is quite good and will last 2-3 days afer charging to 90%. Forget BQ if you want to use Linux - or need service.
   I can not say anything about the problems that were reportet abaut other Linux devices. With a fairphone2 I have no problems using ubuntu touch.

I'm using Ubuntu 16.04 (2018-W48) - as well as older versions.
every time M10 FHD is booted the

• password for WiFi has to be entered again
  so you will get an additional networks like GAST, GAST1, Gast2 ...
• WiFi adress (MAC) has changed
• Bluetooth adress is unchanged
  Also for my M10 the Serial 0123456789ABCDEF is displayed - that not true!
  And: It is not possible to connect the M10 via HDMI with an monitor.
  So I think there is are problems with all I/Os.