I will guess that, if you have done nothing operationally incorrect, that you are the victim of a recent problem with a transition of CVSync versions, which is the tool used worldwide for OpenBSD's AnonCVS mirrors. This will have affected -stable tags (OPENBSD_5_5) as well as -current (HEAD).
The announcement:
http://marc.info/?l=openbsd-announce...6803813019&w=2
The problem may be resolved by now for the AnonCVS mirror you use.
I recommend you contact the maintainer of your mirror directly (the maintainer's Email addresses is listed in
http://www.openbsd.org/anoncvs.html#CVSROOT and ask if it is now safe to run cvs(1) update to obtain a correct working directory in /usr/src.
This is all just a guess, of course.