Pádraig ó Beirn

I had the exact same issue using Firefox & Chrome.

However, Event Calendar works for me using IE9 without having to disable Wordbooker.

Even though I didn’t have to, you may need to switch to development mode in Cloudflare (if you use it) before adding a new event. I don’t use IE at all but came in handy to get around this error, at least for me!

No problem ??

Glad it was useful to somebody else.

Figured it out!
Placing this at the end of my .htacces file didn’t work:
<Files admin-ajax.php>
Order allow,deny
Allow from all
Satisfy any
</Files>

Placing it at the TOP of the .htacess file worked!

I also have similar problem. What did you do?