cvs-execute-single-dir is a variable defined in `pcvs-defs.el'.
Its value is

Whether cvs commands should be executed a directory at a time.
If a list, specifies for which commands the single-dir mode should be used.
If t, single-dir mode should be used for all operations.

CVS versions before 1.10 did not allow passing them arguments in different
directories, so PCL-CVS checks what version you're using to determine
whether to use the new feature or not.
Sadly, even with a new cvs executable, if you connect to an older cvs server
(typically a cvs-1.9 on the server), the old restriction applies. In such
a case the sanity check made by PCL-CVS fails and you will have to manually
set this variable to t (until the cvs server is upgraded).
When the above problem occurs, PCL-CVS should (hopefully) catch cvs' error
message and replace it with a message telling you to change this variable.