Hello @parolnix
I can see that you have replied to several threads, please do not duplicate identical posts in multiple threads.
First of all, you are using an outdated version of Document Server. Please consider updating for better results.
In general, please refer to this thread where similar issue is described for possible solution:
If information there doesn’t help, please provide logs of Document Server from /var/log/onlyoffice/documentserver for analysis here in this thread.