I run into the following issue:
By accident, I entered two times the same category into the default user preference field for the default watched categories.
Result: Accounts could not be created anymore
PG::UniqueViolation (ERROR: duplicate key value violates unique constraint "idx_category_users_u1" DETAIL: Key (user_id, category_id, notification_level)=(16, 11, 3) already exists. )
Solution: I removed the enry from the watch list so that the list was unique.
However, I would expect that Discourse doesn’t allow to add two times the same category to the watch list.