I am having the same issue. Deactivated Ultimate Member and had no issues... I'm going to have to go back a version in wpforo until this is addressed...

There was a similar issue with a plugin called Caldera forms.. something in their update didn't like Ultimate Member... I do not know any details other than it generated almost exactly the same error

I've just put a notice that forums are temporarily unavailable. I will keep watch here for updates. I really don't want to have to try to go backwards...

The UM part is temporary disabled, you can update now. For some reason the most used um_user() function of UM plugin starts looping infinity when you call it in other plugins. In other words UM functions not usable for third party plugins. It was working on our local servers but it isn’t on most hosting servers. So we’re currently working on non-standard integration way. We’ll update the UM integration soon. At the moment you can update wpForo and use it without any issue, the core is just updated.

ok, forgive my ignorance. Will an update come through Wordpress or should I just download using the link on the home page?

Posted by: Robert

The UM part is temporary disabled, you can update now. For some reason the most used um_user() function of UM plugin starts looping infinity when you call it in other plugins. In other words UM functions not usable for third party plugins. It was working on our local servers but it isn’t on most hosting servers. So we’re currently working on non-standard integration way. We’ll update the UM integration soon. At the moment you can update wpForo and use it without any issue, the core is just updated.

@robert DWQA forum has working UM integration. I have a pro version of that plugin which I'm happy to share of this helps ? Perhaps you could gleam some Intel from that ?