Kopano SSL key file permission denied after some days


we are running Kopano and I noticed some behavior I an not understand regarding kopano-gateway. Kopano is configured for IMAPS and works flawless. After approximately two weeks without anyone touching Kopano or the server at all, mail clients can not connect to kopano-gateway because of SSL issues. In kopano-gateway.log I can see the following lines:

Mon Jan 22 06:25:20 2018: [error  ] [23218] SSL CTX private key file error: error:0200100D:system library:fopen:Permission denied
Mon Jan 22 06:41:18 2018: [error  ] [27589] ECChannel::HrEnableTLS(): invalid parameters
Mon Jan 22 06:41:18 2018: [error  ] [27589] Unable to negotiate SSL connection

This happened for the third time now. A simple restart of kopano-gateway solves the issue for a few days but I would like to know what is happening. Does anyone have a clue for me?

Thanks in advance.

Please have a look at Kopano, Kein IMAP-Zugriff nach logrotate

Thank you for the hint. Looks like a solution. I will try it out.