This is no longer an issue on Debian 10 with the upgrade of roundcubemail to 1:1.5.4.11-1~kolab1 and roundcubemail-plugins-kolab to 1:3.5.11-0~kolab1. The context menu is back and I am super grateful.

The referenced ticket T7247 discusses a Kolab installation on Debian 11 (and the various issues that arise).

That is not an option I am afraid, as apt will then propose to downgrade roundcubemail and remove chwala, irony, etc.
This is actually what I meant with the ticket... there is a new package called 'roundcubemail-plugins-kolab' that seems to supersede all individual previous plugins.

My Linux distribution is Debian 10, the roundcubemail-plugin-contextmenu package is no longer available for installation using apt, after you upgraded roundcube from 1:1.4.13-0~kolab3 to 1:1.5.3-1~kolab3 (before it was available and it was actually automatically installed during the kolab installation).

Thank you, it is resolved and along side with it, the folders in settings are now visible and editable again via the UI (path /?_task=settings&_action=folders). Thank you for fixing this, it was rather crippling

Thank you! It is resolved.

Root cause seems to be this: https://git.kolab.org/T7400

Root cause seems to be this: https://git.kolab.org/T7400

In T6061#89371, @armageddon wrote:

Have a look on this repository.

I managed to restore an older backup and retrieve kolab_guam-0.9.7 and eimap-0.4.2 (Debian 9 amd64), and following @transcending.it advice I have my system up and running again. Just in case someone can benefit and save some time, I am uploading them here...

In T6061#88954, @transcending.it wrote:

I uploaded the previous guam 0.9.7-3 and erlang-eimap 0.4.2-2.1 here from DEBIAN 10 for those of you who don't have access anymore. But I strongly suggest to take any software only from trusted sources (i.e. your own machines - or maybe the kolab devs will upload old versions to the kolab repo so we can use apt).

I confirm the behaviour for apache2 as well.