For some reason yesterday started to show this message on wordpress backoffice and also on every registration page. The message goes away in couple seconds but the users aren’t able to register to the event.
We never had cache (server and plugin) activated so the only behavior we had different to the past was to create a couple events with 2 tickets and then disable one of then (so that ticket became locked).
The first thing to check is look for a plugin conflict. This same issue was reported last week and it turned out to be a plugin conflict. Here’s a link to our troubleshooting guide that includes information about how to rule out a conflict with another plugin: