All in all, it took 9 attempts for me until the cover display update was successfully flashed. Four times, it showed an error right at the beginning after some 30s or so. For the rest of the unsuccessful attempts, it just got stuck at some point, the progress bar didn't move on anymore, but no error message appeared. This seems to have been triggered by events on the Cosmo:
- The first time, I didn't know how flaky the flashing was and started doing other stuff on my Cosmo. The update immediately got stuck.
- The second time, it stopped when an email came in. From then on, I used flight mode and do not disturb.
- Not sure about the third time, but I think the "problem" may have been reaching 100% state of charge. Just to be sure, I deactivated Tasker, then.
- The fourth time, it got stuck when I switched off the lights in the room and the display was dimmed down. So I switched off adaptive display brightness. I also took the Cosmo off the mains because I wanted to take it somewhere else with me. I don't think this had an effect, but who knows?
- The fifth time it worked.
It seems to me that whenever something happens in the background that uses a considerable amount of CPU, flashing the cover display fails. I hope this will be improved. Right now, it's a bit like trying to burn CDs under Windows in the nineties.