I was told by Kusurija@cswiki that the server refuses to save an edit repeatedly with following message.
In Czech: "Omlouváme se, ale váš požadavek se nepodařilo zpracovat. Zkuste to prosím znovu. Pokud se tento problém bude opakovat, zkuste se odhlásit a znovu přihlásit"
Translated to English (not exact, can't find the message code): "We are sorry but your request couldn't be processed. Please try it again. If this will repeat, please try logout and login again."
At en.wikt an edit could be saved after loging out and saving as IP address.
I have no problem with saving edit and didn't noticed any other user with this problem.