The same issue for me.
The server is new to me, it seems they have done migrations earlier.
This is RHEL7.5, KNIME Server 4.9.1
The error appears regularly in apache-tomee-plus-7.0.3/logs/localhost..log
19-Dec-2019 12:03:18.166 SEVERE [https-jsse-nio2-8443-exec-154] com.knime.enterprise.server.application.repository.RepositoryTreeTranslator.addItemsRecursively Error while building repository tree for user ‘username’ at ‘/Public/Email_test/2018-12-20T09-42-50.555-0500’ : Owner must not be null… Some items may be missing in the tree. The message appears a few thousands times per day.
The username one of 7 names, including me, the date looks suspicious because it is too far in past - a year ago, and I definitely didn’t work with it earlier.
We have a workflow /Public/Email_test, but it has no any mention of this strange date.
From the previous discussion I see that the idea is to check shared metanodes permissions, and there is a metanode in the workflow, but I can’t find how to check it’s permissions. Permissions of the workflow itself looks good.