Thanks for answering. I also noted that the UI flashed "starting array" repeatedly in the status line, even though the array was already running and accessable from other machines. I guess not being able to read from the boot flash device confused the ui completely. If rebooting is fine, i figured a shutdown would be even better, as i might alvage some of the flash data. I proceeded like this:
1) I shutdown the machine, which was only partly successfull. Telnet, network and all other services seemed to have stopped but the machine was still running after 15 minutes. So i did a hard power off.
2) I put the USB-Stick in a windows machine, it said it had errors. I was still able to access all files and made a backup.
3) Then i used the windows check and repair function which ironically finished by stating no errors found. I used the safe remove feature and unplugged the stick.
4) i plugged it back in again to check if windows would still find errors. It did not. Then i checked some of the files against a backup and everything seemed fine.
5) I plugged the stick back into the Unraid server (same usb port as its the only one) and turned the machine on.
6) Everything seems normal, except for the unsafe shutdown and a parity check which is running now.
7) The plugins i tried to update when the whole incident startet seemed to not have written anything to the boot device, both plugins had the update available again through the normal update check. I updated both without any problems.
So the problem seems solved, but it might only be a matter of time before the usb stick fails again. I will try to get a new one in the next days and transfer everything to the new stick. Is there anything i should know/do in advance to make the license transfer as smooth as possible.
Also i think that unraid should be made a little bit more resilient and graceful if a flash device error occurs. For example if the error message flash device error is displayed why does it still try to access the boot device from numerous pages of the webui? Also it should probably not try to start the array constantly in such cases (or at least not display the message in the status line). And finally a shutdown should still be possible. I know its probably a rare failure - my first of this kind in years with two servers - but everything that makes unraid more robust is welcome.
