There are many users who have added custom functions and codes in functions.php file of their active WordPress theme in order to integrate wpForo with different plugins. If you got Fatal Error or Server Error 503 after updating to 1.9.x version, please check your functions.php file and remove all scripts related to wpForo (in any case, save those scripts in some txt file).
Here is an example of such script which was added to one of our users functions.php to integrate wpForo with UM plugin and it caused lots of problems: https://wpforo.com/community/postid/16663/
All custom scripts related to UM plugin integration should be removed from your active WordPress theme functions.php file. If you've used Code Snippets plugin for adding such codes, please check it too. Currently, you can use the "Profile Page" option in Dashboard > Forums > Settings > Features Tab and switch all profile URLs to UM profile page.