The user selection combo did not react on first time (maybe it only lacks a notification message informing the user that the list is being retrieved? Currently one thinks it does not work, clicks several times, refresh the page, etc.)
A backend error is returned (about a missing "enter" method) and the new user rights are not updated in the remote DBMS.
About the first point, actually this is what happens:
1) Click on the combo to select collaborators
2) The combo opens up with the list of users (OK)
3) Click on one of the users
4) Two cases:
Sometimes, this click works fine (add a check mark next to this user and update the combo value)
Sometimes, this click closes the combo list and does nothing (no check mark, user not added to the combo value). When this happens, refreshing the page may restore the expected behaviour.
Here is a video showing issue (first point)
pb.zip
This is very random, sometimes when selecting another dataset it works, even without a page refresh.