Unable to add other users to proposal as Admin and submiter when editing

[ddemaio]

I'm submitting a ..

• [ X] Bug Report
• Feature Request

Users cannot add additional users to proposals. This also is not possible through the admin interface. Results provide an error seen below.

Expected correct behavior:
Admins and those to submit a talk/proposal should be able to add users to the proposal as some talks have more than one person.

Steps to reproduce:

Navigate to the proposal link and click edit
Attempt to add users in the speaker
Observe the error preventing the action from completing.

Other information:

Issue experienced by multiple users, including admin accounts.

[hennevogel]

duplicate of #2984

there is a temporary fix applied in production now