One quick thing to check is if you can modify tickets when you start fresh with a new event (click Add New on the Events page). If you can modify tickets when it’s a new event, then the problem is related to your site’s web server hitting its max_input_vars limit. The way forward will be to contact your web host and ask them to raise that limit. For example if the limit is 1000, they could raise the limit to 2000.
If you cannot edit tickets on a new event either, then this could be a plugin conflict and you can follow this guide to troubleshoot:
The support post ‘Cannot Modify Any Tickets on Active Events’ is closed to new replies.
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Support forum for Event Espresso 3 and Event Espresso 4.