Client Access for DOS - license problem

Hi all,
we have two machines at V4R5, one at C2169450, the other
 at C2050450 PTF level.
We have 5 DOS Machines communicating with PC Support
Extended (Client Access / DOS is this XA1 or XL1 ?).
Out of a sudden, we cannot access the newer machine anymore
although "nobody changed a thing".
The log complains about a license error, because 5763XY1 is 
not found, and indeed, it is not on the system and has never been.
I reinstalled XA1 and XL1 because somebody had the idea to zip
the folders "because they are no longer needed", got rid of the
errors in "dsplicpgm".
As far as I understand it, one does only need XW1 to get a connection
license to use a licensed component of any ClientAccess flavour.
This has never been touched, but it only works on the "older" machine.
Strange enough, I get the same errors in the log, but this machine 
goes on in processing ..., while the other just stops and the PC hangs.

Do I need XY1 installed on the system ?
What is the coexistence in XA1 (family code base)
XY1 (family code base refresh for Version 4  ??)
and XW1 / XE1 for, where are the licenses taken from ?

Any help appreciated, Philipp Rusch