I was doing several rollbacks and also manual update so USB was reseated several times. Using internal USB header, so shouldn't be a problem with that. I can try different flash, the question is if those errors are for sure regarding flash drive and not something else plugged in into USB, like my UPS or something.
Booting up in CLI and using slim command does not work. CLI boots up no problem, services start, but then when I use slim command it hangs on black screen with "_"
Hmmm to be honest out of everything I have no tried safe mode, because I figured out that if CLI mode works just fine and only GUI is problem then safe mode wouldn't do anything good here. Will give it a shot on the afternoon.