by Juno Sprite.
So I found some more stuff out and a workaround.
Workaround
I created another backup of the 1st Course, called 1st Course - Workaround, and I restored it as a new course into a top-level category (we'll call that Category 2 to be consistent with the example above). I then imported a Topic into 2nd Course from the new course I had just made, and it worked! I did it two more times with the two following Topics, and there were no issues. Wooo! So in this particular instance, you should import content from 1st Course - Workaround, not 1st Course . If that issue arises again with any other course, just re-backup the course you're trying to get stuff from, restore that newly backed up course as a new course (so as not to overwrite anything), then import the stuff you're trying to get from that newly created course.On my server, there should only be one (1) full course backup process happening at a time; otherwise, I feel like more issues like this could arise.
Potential causes
1. It looks like you can perform a course restore from within a course or from Site administration > Courses. I would hope there is not a difference, but who knows?
2. I noticed that two admins were both trying to import content into the same course, presumably from the same course as well. If that was the case, and they both happened to be attempting those imports around the same time of day, could that have caused it, since there was some sort of category ID mismatch?
3. I noticed while monitoring the error logs that the log activity was really delayed; that, or there were just multiple backup/restore processes happening at the same time and the server was trying to catch up, or was resulting in some unfinished processes altogether. Maybe?
Please reply to this if you think you know what happened here. At least it's working for now, though!