@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!

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.

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.

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

@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.