@Er.Pe Probably the solved problem isn't really solved. To be honest, I am not sure, if texts now are coming in without any failure. There seem to be still messages sent, but not shown on my device. Although I am not able to quantify the amount, because I receive text messages (SMS) not every day. I will give notice, if I should get to know more details.

The same thing happened to my OnePlus Nord N10. It got stuck at the bootloader screen after I upgraded from Ubuntu Touch 20.04 to 24.04. I solved it as @MrT10001 suggested: Started Ubuntu Touch in recovery mode Enabled Fastboot and restarted the devices. Installed Ubuntu Touch 24.04 via the installer desktop application. After that, the problem was fixed, and my device is now running 24.04 without any problems.

SUCCESS! Hard to find out what's been the essential step. I just booted OxygenOs 10.5.7 this time with a SIM-card inserted and connected to WiFi immediately. Surprisingly now I could enable OEM-unlocking and after that "fastboot oem unlocked" without any issues. UT 20.04 is running, so the device will be my daily companion from now on. Thank you so much, mates! Didn't find out to mark that thread as solved.