After updating a few plugins today, our WPMS instance which included User Switching 1.0.9 started producing 500 errors. The plugins updated were:
OnTheGoSystems' Access and Types (two separate plugins)
WPMUdev's Appointments+
Alberto Lau's Styles with Shortcodes
WooThemes' WooCommerce Product Add-ons
Note that User Switching wasn't one of the updated plugins. I can't say which plugin is causing the conflict, but given the error log entry, my guess is Access (and Appointments+ isn't active on the site in question).
The error.log entries were of the form:
PHP Catchable fatal error: Argument 1 passed to user_switching::filter_user_has_cap() must be of the type array, boolean given in plugins/user-switching/user-switching.php on line 733
After removing User Switching, normal function was restored.