Not able to add roles to users in privileges view

[timo0]

I'm logged in as owner and I'm trying to add other users in the privileges view. After I select the user in the textfield nothing happens and the user isn'd added to the role.

I don't see any error on the server log. But in the browser console the following error is shown:

Failed to load resource: the server responded with a status of 500 (HTTP/2.0 500)
Error: HTTP/2.0 500
(anonymous function) — admin.js:59634
emit — admin.js:14495
onreadystatechange — admin.js:59768
Unhandled Promise Rejection: Error: HTTP/2.0 500�

My current workaround is to edit the object in forums manually and add a entry for the permissions array.

{
    "user": ObjectId("foo"),
    "role" : "admin"
}