So...error persists after unlocking topic prior to move.
Yet, moving the only post in the topic to a new topic in another forum proves successful, with removal of the empty topic in the source forum. (Hope that makes sense.) So, this is our workaround since it cannot be replicated.
No new error in the SP log though, perhaps because it was identical. Looks like timestamp on error may have been updated.
Reassigned user of first post and issue persisted upon attempted move.
Noting special about topic title.