Sorry to hear this Timothy!
Our plugins would not generate this sort of error, and so it may be something arising from a server issue or an issue with your theme or other plugins on your site.
Here’s what I would recommend doing:
1. Contact your web host’s support team to see if they have any insight on this. There may be a server or .htaccess issue causing this, and they can tell if that’s the case and/or offer a solution.
2. If that’s not fruitful, do the complete set of troubleshooting steps on this page → https://theeventscalendar.com/knowledgebase/testing-for-conflicts/ After EACH STEP in that process, go back and try to “make event post or work on any old events” and see if the error goes away anywhere along in the process.
Let us know what you find! 😀
— George