When creating a Phish Threat campaign, it will sometimes hang at the Enroll Users page when trying to enroll a big group of users.
This behavior may be experienced when a high number of groups are imported into Central and most imported users are members of multiple of those groups.
A possible workaround for this is to remove all the groups that are not needed from Central - keeping the list as short as possible. If AD Sync is in use, the AD Filters would need to be modified so that only the necessary groups are imported.
We are looking into a more permanent solution, but the above should allow for the campaign to be created and sent out.