Wp goes in maintenance mode and thats it all.
I would suggest you check this article: https://kinsta.com/knowledgebase/wordpress-stuck-in-maintenance-mode/
It should be helpful for you.
I know how to fix it dear, wha i dont know is how to update the WPForo.
Please read the article. The maintenance mode is not controlled from wpForo. Your hosting or WP has some difference so somehow it doesn't remove the maintenance mode file after the update. It may be file system permission issue as well:
In other words wpForo update goes well, the problem is in your website and hosting server. The switching to maintenance mode for update processe and back switching to normal mode causes some problems.
So just try again and follow the instruction in the article. Remove the mentioned file if maintenance mode is not disabled automatically.
The maintenance mode is not controlled from wpForo.
And i repeat, i know that, i fixed it already.
Ok, then please contact to your hosting service. If your server has such an issue and you have to delete that file then something is wrong with your server. So it would be better to ask the hosting service to help you. This cannot be fixed from wpForo side.
Also, try to find some error.log file. It might be very helpful.