Ok. Apparently Archicad thinks the file that it has opened is read-only, so when you select the Save command, it defaults to Save As. I you're working in Teamwork, try to isolate the problem by creating a Solo project (.pln file), save it to the server, open it remotely like you describe, and save again. If the same issue occurs again, it's not a Teamwork problem.
It might be a bug. I'd contact the reseller and ask for support.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1