IMAP problem after upgrade to eM Client 7


I just updated my eM Client from version 6 to 7.
eM Client 7 imported all the mails, calendars and contacts from eM Client 6.
But then the fetching of the e-mails from my mail server failed with the message “Der Server antwortet nicht” (Server not responding).
I’m using a cyrus mail server, with a self certified SSL certificate on the port 993.
It is configured as “Sicherheitsrichtlinie: SSL/TLS auf bestimmtem Port verwenden (Legacy)” (Security policy: Use SSL/TLS on special port (legacy)).
The SMTP server is working and it is using the same certificate.
I deleted all user data and reconfigured the e-mail stuff from the scratch but still the same issue.
I tried to enable the logs, but they all stay empty.
The only content I could find was in the log file without suffix:

19:39:19.301|00F|   System.Net Information: 0 : [6224] InitializeSecurityContext(credential = System.Net.SafeFreeCredential\_SECURITY, context = 534d70:8b624f0, targetName = XXXXX.XX, inFlags = ReplayDetect, SequenceDetect, Confidentiality, AllocateMemory, InitManualCredValidation)

19:39:19.302|00F|   System.Net Information: 0 : [6224] InitializeSecurityContext(Anzahl von In-Buffers = 2, Länge von Out-Buffer = 0, zurückgegebener Code = InvalidToken).

How can I solve this problem?

The server is working correctly, I checked it with my mobile phone, tablet and also with a downgrade to eM Client 6.