Can't remove topics

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I am making some changes to a site, and I need to delete the existing topics and replace them with new ones. If I delete the topic tress, all the pages, even once that had no topics, say

Load error! (Internal Server Error)


in the topic field in attributes.

Looking the logs I see
Exception Occurred: /storage/sites/client/concrete/src/Permission/Checker.php:81 Unable to get permission key for view_tree_node (0)


But after some hours of trying to figure out what that means, and how to fix it, I have come up empty.

Does anyone have any insight on this?

I also tried to delete all the tags topics from every page first, then all the categories, then all the topics, in ascending order, before deleting the topic tree, but it made no difference.

Thanks

ntisithoj