It may be related to Group mention prevents editing topic. I have upgraded to v1.9.0.beta12 +5 recently and removed the suspected plugin, but keep having error 500. Group mentions seem to work (I do have groups popping up when using @, and the group members seem to be receiving the notifications.)
The settings for all the automatic groups have changed to nobody also on my forum.
I had already changed the settings on “only admins and mods”, so they were reset to “nobody” after the update.
For custom groups at a certain point I suspect that the permission on Meta have been changed from “all” to “only group members, moderators and admins”, that’s why I can see and mention “translators” but not the other custom groups.
Regardless of the setting, I’m sure that in the past all the groups were visible when you typed them on the editor.
When someone who did not have the permissions typed the name of the group a popup warning that no notifications would be sent to the members of that group.
Ahh this might be my “bug”. I “fixed” it such that if a group is not mentionable, they wouldn’t appear in the autocomplete. I found it weird that you would be able to autocomplete a group but not have it become a mention