The issue appears to have been the fact that the site does not accept keywords/tags that are 2 characters long and it also dislikes keywords that are more than 15 characters long. While it would be nice to be able to enter one of my characters names proper (TJ), the workaround is simply not to use small or too long tags.

I just tested my previous theory. It seems that there's the thing with 2-character keywords, but 15+ characters appear to give the same issue. I'm not sure exactly what the cutoff is, somewhere between 15 and 20. So to be sure, Keywords apparently need to be 2-15 characters long.

The problem arises on submissions without those keywords. However, on FA I did like to add a small phrase as a keyword as well written_like_this_format. Apparently, removing that from one submission to test solved the issue.

The error seems to happen with the short and 'too long' keywords.