Wow. This is the issue. The pre-seeded categories thing doesn’t exist, the text color was set to #FFFFFF in this one category.
If I set #FFFFFF as the text color category, then the problem vanishes.
/admin/logs/staff_action_logs shows that I never set the text colors to #FFF. It was the default value. It was a migration from vBulletin to Discourse, if it’s of some importance.
Perhaps the import script set the value to #FFF, avoiding the validation script and creating this bug (though I’m not sure at all import scripts set colors…). 