I am 99% sure I invited new members in 1.3.6. Yesterday I updated to 1.3.7 because of the Firefox extension issue that is now solved.
Unfortunately I can now no longer add any users. I tried with different users, different organisations, nothing worked.
The user will get the invitation mail, but when clicking on the link, the login page will show this error:
Tested it with multiple users, multiple browser, incognito mode.
