@mike Okay, now that I'm back up and running, I have the fix for that issue to pass along:
As a user of a node/instance:
Settings ➡️ Additional features ➡️ Post Composition Features
then check: "Explicit Mentions"
(this will resolve it for the individual user)
As a node/instance administrator:
Admin ➡️ Additional features ➡️ Post Composition Features
then check both: "Explicit Mentions" and "Lock Feature Explicit Mentions"
(this will solve it instance-wide)