X server dies starting kde

kdm ... X server died during startup... X server for display :0 cannot be started, session disabled

It used to work fine on 10.3, but since upgraded to 11.0 I get only the above message.

What to do?
I bet Sevendogs nailed it. I had the same problem. Also, make sure that KDE is reinstalled after Xorg (if it's not already updated).
After the upgrade to 11, did you also upgrade all of your userland software (ports/packages)? The upgrade to 11 only updates the core OS, not the user installed software, of which the X server is part. Read this part of this page: " Upgrading Packages After a Major Version Upgrade" https://www.freebsd.org/doc/handbook/updating-upgrading-freebsdupdate.html.

Many thanks. I thought I had done it, but apparently not! Now all is fine again -- apart from virtual-box-ose-additions.
Glad you got it resolved, happy to help. Don't forget to mark the thread as "solved" by using the "thread tools" button at the top, right of the page.