Looking a bit more at my configuration and trying to find something relevant:
I think one of the admin changed the visibility from “Everyone” to “Logged on users”
and now the "Allow users to send membership is checked, but disabled
When I’m looking at another group (visible only to logged in users)
The “allow users to send membership requests” is disabled and unchecked
Could it be the issue? is this expected that you can’t send membership requests to a group that is visible only to logged in users?