hm, apparently this error is
should be (TM) fixable by deleting "clientregistry.blob" in you Steam directory (with Steam closed obviously).
(for example according to
this (german) thread);
(which also states that you should generally open Steam
before the Enderal launcher, and play offline,
don't know, if that does..
something 
)
hope that helps
EDIT:
I'm not sure, what deleting that file does, but it's completly possible that you have to login into Steam again,
so maybe don't do that without having your password ready
