Please go to your Usergroup Manager and be sure that you have entered in a forumid for the usergroups that you wish to enable auto subscriptions for.
In version 2.4 you could actually turn on auto subscription for a usergroup and not enter in the forumids. This would spiral to a few other errors such as the one you mentioned above
2.5 will not allow you to not enter in a forumid if you are turning on auto subscription. So I'm fairly certain that when you installed 2.4 that mistake was made.
I've put in a safety net to ensure that fumble isn't made with these versions going forward