Trying to perform an illegal action

would it be possible to try deactivating and then reactivating EM plugin again?

Yeah – tried that – no luck! I have even tried to reinstall.. and deactivating all other plugins, and change the Theme… again = No Luck!

On which exact page are you seeing this error?

I know the page is in Danish – but you can see the error here:

https://5935e40f832db0c1361f013694c67a40f7fd2edc.web15.temporaryurl.org/events/1-august-2/

I see now how I think it’s a plugin or theme conflict since this works on out test sites and the ajax when submitting the booking. although you already did try disabling plugins and theme maybe you can try it again by

1. deactivating all other plugins except for EM
2. switching to the default theme
3. try to book again

I getting same issue, i disabled all other plugins. I don;t think it is the theme, as I getting this on the admin side and using default theme.

Any other ideas??

Have you tried switching to the default WordPress theme? Sometimes themes can interfere with both the front-end and admin areas of WordPress.