inviting new users no longer working (admin only)

[eplebel]

steps (from admin account):

1. create new author (avatar down caret --> "Create author page")
2. go to Invite page (avatar down caret --> "Invite new users")
3. keep radio button selected to "Invite user with an existing author page", enter valid & unique email address
4. enter username of new author (from step 1)
5. click "SEND INVITE"

result: Nothing happens, and console shows a "500 error"

previously, this issue would be caused by the fact that a security option "Less secure app access" was changed to "OFF" in our gmail account (curatescience@gmail.com), and turning it back to "ON", and then sending invite WOULD work

but now, even with that "Less secure app access" security option turned "ON" in our gmail account, we're still somehow getting the "500 error"