it looks that the case to this issue has it's origin in a renamed webspace. We had one for a while, where we changed the key at some point.
What is the correct way in sulu to savely delete or rename a webspace?
Thx a lot!
As pointed out by @Alex in this issue, the solution is something like:
bin/adminconsole doctrine:phpcr:node:remove /cmf/oldwebspacekey
bin/websiteconsole doctrine:phpcr:node:remove /cmf/oldwebspacekey
A backup before running this commands is recommended, but it seems to work rather stable.