Content in Panel (randomly) not saving; user logged out or cannot log out without refreshing

Tested also in other browser, but there I everytime get logged out when I hit refresh … in both consoles, there is this error:

Quellübergreifende (Cross-Origin) Anfrage blockiert: Die Gleiche-Quelle-Regel verbietet das Lesen der externen Ressource auf http://localhost:3123/socket.io/?EIO=3&transport=polling&t=Md4xNdT. (Grund: CORS-Anschlag schlug fehl).[Weitere Informationen]

And sometimes the siteoptions (yes, site.txt) are working, sometimes not …