It sounds like the edit is going through with some HTML in the ConditionComments field... The edit should be against a single record, so if you can identify that record, you can determine if the field already has the HTML in it, and for some reason now returning an error about it, or someone is adding HTML to the field during an edit operation. I thought you would be able to see the edit parameters in the DEBUG logs.
User here was editing a field but another ready only field ended up being the culprit. This Invalid html content in json request body solution resolved my issue of "Invalid untrusted Html content detected in field". Once web filter web content was disabled and service was restarted user was able to sync in Collector.