Nullpointer exception during build

W2k hosted IDE 620A.

starting build or rebuild in a C project, the following error msg comes up:

Errors during build:
java.lang.NullPointerException encountered while running
com.qnx.tools.ide.cdt.make.internal.core.MakeBuilder

It was working for a while, but now after each build this msg comes up.
Any idea ?
peter