What this means: The job that adds Campaign Members from Eventbrite attendees stopped with an error it could not recover from, so some attendees may be missing from their Campaign.

Common causes: Validation rules, triggers or flows on Campaign Member; another managed package hitting a governor limit; the Campaign Member Status limit being reached.

What to do: Open Maintenance for the full message. The text in brackets after "U:" shows which package or code threw the error, which is often not ours. Check Apex Jobs in Setup, fix the cause, then run a Full sync.