Problem and question: Java webstarted app looking for its classes in base folder instead of ./lib.
As suggested in similar question at Java Web Start applications a
The "mydebian.mydomain" hostname hints you are running on Debian, where OpenJDK is the default Java implementation.
The Sun Java WebStart implementation is not part of OpenJDK, so an alternative implementation is used. As Java WebStart does not have an official TCK there are subtle differences and even bugs in the OpenJDK implementation which needs to be found manually (as there is no official TCK).
I would suggest trying with a Sun JVM (e.g. on Windows) and see if the behaviour changes.