Why am I getting the error "A serious error occurred please contact the administrator" when using the Experience Editor in Sitecore?
Anything which alters URLs e.g. URL rewrites etc could potentially cause this error. Also, we had some code which was generating a new anti-csrf token which caused the above error.
Sitecore logs are sometimes not very helpful, in which case check the browser console window for any failed AJAX requests which can provide further info.