@Bolly I wish. See here.

@domubpkm Please read my post and the thread to avoid double posting in the future.
Because the device was stuck in a bootloop and all standard measures to shut it down proved futile, I had to physically open the device and disconnect the little power cable on the board. (I'm ruthless when it comes to hardware. Only do this at your own discretion!) After that I was finally able to boot the device to turn on developer mode. Since the screen was flickering really badly, it was hard to see everything but it worked. After that I re-flashed the device including a data wipe to OTA-25 stable, because downgrading is more or less impossible. However, I still get random bootloops on the MH 10. @aimar maybe re-flashing your device is currently your best option.

@lduboeuf Guess what, I just tried to boot the Cooler and now it's not only stuck in a bootloop, I can't even break the loop with power + volume- which means I can't even shut it down anymore. As a result, I can neither switch to developer mode to flash it with an older build nor can I select the bootloader from the menu because power + volume+ doesn't work.

@Flohack do you have any ideas how to get the tablet to shut down? Currently, I could only hope for the battery to give out. Unfortunately, I can't seem to register a bug, because my atlassian account has no access to ubport's issue tracker.
Maybe BQ devices should be announced as not supported by OTA-25 until the issue is fixed.

@domubpkm Thanks for your post! Believe it or not I will have to try this right now. I just tried to boot the Cooler and now it's not only stuck in a bootloop, I can't even break the loop with powerbutton + volume- which means I can't even shut it off anymore. I've never encountered anything like it before...

hey! Look at this thread: https://forums.ubports.com/topic/8750/ota-25-call-for-testing-companion-post - yes it seems to be a general problem with BQ devices.
If you can downgrade back to OTA24 that's probably the best (or: only?) solution at the moment. Tbh, I'm kind of expecting my BQ M10 HD to stop booting altogether because it's been struggeling with bootloops since upgrading to OTA25.
best of luck!
Edit: I should add that downgrading is purely a quick-fix idea from user to user. I'm not really familiar with the inner workings of UBports.

@Ællen Switching to stable solved the problem for a day and then it came back. Still/again need 2 attempts to boot the bq cooler.

Same here: since the RC updates on Mar 16, then Mar 23 and Mar 26 my bq aquaris m10 hd Ubuntu Edition won't boot properly. It starts booting then seems to stop after the initial splash screen, sometimes after the next splash. Then I have to shut it down and restart. Usually the second time around it will boot properly.
I'm considering switching to stable to see if that fixes the problem. I'll post here if it worked.