Pretty sure you're mixing content-translated and non-content-translated databases again. This is trying to install WITH content translation against a forum backend WITHOUT it.

Not the case, both sites were set to support content translations this time around.

I just tested myself. I did find an entirely unrelated bug, but could not reproduce this. I'm fairly sure I'm right too. This is the error:

Unknown column 'f_description__text_parsed' in 'field list'

This column exists on a content translated site only, so it being missing means the central site does not have content translation.
There's no way I can imagine for it to be missing due to some other bug, as the consequences of that would be severe.