Any ideas?

Sessions expire as always.
Check your settings in /etc/cron.d/sogo
There is a line with

* * * * *      sogo    /usr/sbin/sogo-tool expire-sessions ...

Is that line commented out?
If not, which number is behind expire-sessions?
That number defines how long a session is valid without the user sending any input/request to the sogo server.
Default is 60 (minutes).
Therefore if a user is writing on that email more than 60 minutes he/she will be logged out.

After login you can find draft messages in the "Drafts" folder, therefore not loosing all that was written.
The intervall for writing drafts to the Drafts folder is set in "Settings" --> "Mail" --> "General" --> "Auto save every XXX minutes".
Default is 5 minutes.

Similar issue here:

When creating a new mail, the session is terminated exactly after 5 minutes.

It does not matter if one sends an event (key, mouse) before, the session is terminated.

There is no parameter to set the timeout systemwide or for individual users.

Workaround: Set the Autosave to 1min to regularly save drafts.

in /etc/cron.d/sogo it says:

# Session cleanup - runs every minute
#   - Ajust the nbMinutes parameter to suit your needs
# Example: Sessions without activity since 60 minutes will be dropped:
#* * * * *      sogo    /usr/sbin/sogo-tool expire-sessions 60 > /dev/null 2>&1

Session expiration is commented out, so this does not run.

The timeout must be handled somewhere differently.

You did run /usr/share/doc/sogo/sql-update-5.5.1_to_5.6.0.sh after upgrade?

After updating to version 5.6.0 the problem disappeared.
Also uncommented the line.

* * * * *      sogo /usr/sbin/sogo-tool expire-sessions 60 > /dev/null 2>&1

In any case, everything is fine now! Thanks!