[sylpheed:37217] Could not get OAuth2 token.

Hello,

today my sylpheed stopped working with the IMAP-server at mailbox.org with this messages:

[10:04:51] IMAP4< * CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE UIDPLUS LITERAL+ AUTH=PLAIN AUTH=LOGIN AUTH=OAUTHBEARER AUTH=XOAUTH2
[10:04:51] IMAP4< 3 OK Pre-login capabilities listed, post-login capabilities have more.
[10:04:51] IMAP4> 4 AUTHENTICATE XOAUTH2
[10:04:51] IMAP4< + 
** LibSylph-WARNING: [10:04:51] Could not get OAuth2 token.

** Could not get OAuth2 token.
[10:04:51] IMAP4> 5 LOGOUT
[10:04:51] IMAP4< 4 BAD Invalid base64 data in continued response
** LibSylph-WARNING: [10:04:51] Could not establish IMAP connection.

** Could not establish IMAP connection.

Where is the problem located? Sylpheed od IMAP-server?
What can be done to make my sylpheed working again?
Can sylpheed be forced to use PLAIN authentication?


Greetings, Frank