Those are warnings, not errors. They don’t necessarily mean anything is wrong, they’re there to give you more information about what’s going on. There could be a number of reasons why content from the Fediverse is not processed, many of which are to do with the person sending the content. If you don’t want them showing up in your logs turn off verbose logging for the ActivityPub plugin (activity pub verbose logging
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Is the topic in an ActivityPub category or does it have an ActivityPub tag?
Was the topic created in an ActivityPub category or with an ActivityPub tag?
Please note that changing the owner of a topics is intentionally disabled for ActivityPub topics. As to why this is the case, please see above: