Invite reviewer only works after the selection dropdown is modified
When entering Manuscripts > Control > Manage reviewers, the "Invite reviewers" dropdown initially shows a selected user. If you hit the Invite reviewer button without modifying anything, it fails to invite the selected user and the button becomes disabled.
Workaround: open the drop-down list and select the desired user (even if it's the same user as currently selected). Then it will work.
It's probably preferable that no user is selected from the outset.