After making the switch from Java 7 to Java 8, and from proguard 4.9 to 5.0beta2, I can no longer optimize my project due to the error mentioned in the title.
I did not find a way to get the actual exception stack, if you like to enlighten me :-)
The maybe relevant program output looks like this:
You should check your configuration for typos. (http://proguard.sourceforge.net/manual/troubleshooting.html#unknownclass) Note: there were 3 unkept descriptor classes in kept class members. You should consider explicitly keeping the mentioned classes (using '-keep'). (http://proguard.sourceforge.net/manual/troubleshooting.html#descriptorclass) Note: there were 4 unresolved dynamic references to classes or interfaces. You should check if you need to specify additional program jars. (http://proguard.sourceforge.net/manual/troubleshooting.html#dynamicalclass) Note: there were 11 accesses to class members by means of introspection. You should consider explicitly keeping the mentioned class members (using '-keep' or '-keepclassmembers'). (http://proguard.sourceforge.net/manual/troubleshooting.html#dynamicalclassmember) Ignoring unused library classes... Original number of library classes: 44120 Final number of library classes: 44120 Printing kept classes, fields, and methods... Shrinking... Removing unused program classes and class elements... Original number of program classes: 4692 Final number of program classes: 4163 Inlining subroutines... Optimizing... Method must be overridden in [proguard.optimize.DuplicateInitializerInvocationFixer] if ever called~~~~