Hi Andy, thanks for your help. Below you can find my answers to your steps:
1. No, I am the only one.
2. Yes, I have the latest version installed.
3. I have already tried that yesterday, didn't work unfortunately.
3. None of those boundaries are met, I'm just trying to sync a small number of files.
4. Dropbox is installed on my Mac OS, but not on my Parallels VM / Windows system. Can that still matter?
5. I don't understand what you mean by "using your admin account"? I'm a user not an admin. And how can I reproduce the issue if I don't know what caused it? :)
Thanks a lot for any further ideas.
Kind regards, Hidde