Hi everybody,
I have made some attempts and found the following:
The first vault can be synchronized with iCloud. The second vault, which is to be synchronized with iCloud, causes the "Authentication Error" when logging in. Here the browser is started and you have to enter username and password. This is not necessary with the first vault. I assume that the error has to be found with the authentication via website idmsa.apple.com.
I hope that this is helpful to correct the error.
Thanks,
Klaus