phpBB3 > MyBB Merge Fatal Error: Uncaught exception

[Jones H]

Your main forums are forums, which can't be displayed by MyBB. You need to go to your ACP -> Forums and either change them to categories (which may not always work as some of them contain threads) or create a new parent category. That's one of the issues I mentioned before:

[...] you'll see some other issues as your database contains some old values which aren't considered (eg most accounts were unapproved or your forums didn't have a parent category).