As discovered in MDLQA-10571, if you set up the Google OAuth 2 service for Google and enable the Google Drive repository then go to add a file from Google Drive as a forum post attachment, when you try to log in, the following error message is displayed:

Unknown action!

Debug info:
Error code: unknowaction

Stack trace:
line 496 of /lib/setuplib.php: moodle_exception thrown
line 531 of /mod/forum/post.php: call to print_error()