We run Jira Service Desk (4.1.0), Core and Software (8.1.0) on premise on a Ubuntu 16.04 server.
Https (ssl) is configured according to the official guides (iptables redirect to 8443) and it is been working fine.
We use MS-SQL2014 as database server.
We access Jira with the base URL.
However today a user discovered that he could not comment on a Jira Software issues and got the error message below:

I have checked all of the Jira logs without any error message, including:
atlassian-jira.log
catalina.out
I have tried to comment on several issues without any success, my user has the correct Jira permissions.
Could you please assist?
Regards
Jon