Hello aka_busker.
Many thanks for the answer. What I've done so far:
I renewed my password (online) again. I can log in online without any problems.
I downloaded StudioOne again, uninstalled the old one and installed the new one. I still cannot log in with the program. I started StudioOne as an administrator with zero success. I also checked the firewall. StudioOne is coming through.
Addendum: I think I found the problem - I simplified my password (fewer special characters). It works now.