This error means that the application is attempting to work with a database connection that has already been closed.
It most commonly occurs after a period of inactivity — for example, when you have had a window or tab open for a long time and the database connection has expired in the meantime.
Typical scenarios:
a document or agenda left open for a long time without any activity
resuming work after an extended break
a connection interruption (e.g. network issue, server restart)
How to resolve the issue:
close the affected window or tab
reopen the agenda / document
if needed, log out and log back in to the application