Problems with Fastmail and the error-message: "System.OverflowException: Der Wert für einen UInt64 war zu groß oder zu klein""

I upgraded my Version 6 to Version 7. After I make a click in my Fastmail-Folder the client crashed. The error-message was: “System.OverflowException: Der Wert für einen UInt64 war zu groß oder zu klein” in english: the value UInt64 is to big or to short. I don’t know where the problem is.
I have deleted the account and do many other things but my problem is still the same. Is there someone with an Idea?

best regards
dubidua