Not sure where to put this - hope it is useful to someone
If you download and install the Mail app for Nextcloud - an experimental app not included in the Nethserver modules install - and you have problems reading email bodies, this may be the cause.
In the browser console, you’ll get a message saying something about loading files from http://yourserver/… violating the content security policy. If you log out of nextcloud, and add
'overwriteprotocol' => 'https',
to the nextcloud/config/config.php file, it seems to solve the problem.
I could not see where the content security policy was being set, though. And this is better than relaxing the security policy anyway