The cause of the internal error was the following:
We have a generic in an unit ("A") which uses another unit ("B") in the implementation section and this other unit uses unit A in the interface section. Now the generic is specialized in the interface section of B. This leads to the problem that in unit A when it tries to load the globalsymtable of unit B that globalsymtable will be Nil, because parsing of the interface section is not yet finished. Thus the change in pgenutil.pas, specialization_init: if the unit is still "in_interface" the localsymtable needs to be used instead of the globalsymtable.

This doesn't necessarily lead to a compiling test though, as there is the following possibility:
Unit A contains a generic class/record (with methods) and uses unit B in the implementation section. This unit B also contains a generic class/record (with methods) and uses unit A in the implementation section. Both units contain a specialization of the other unit's generic outside of it's...