java.lang.ClassFormatError: Illegal class modifiers in class <class name>: 0x419

While porting my application from Scala 2.8.1 to Scala 2.9.0-1, I run
into the following runtime error:

java.lang.ClassFormatError: Illegal class modifiers in class : 0x419

The project is built through maven/eclipse.

It's hard to track down exactly how and where this error occurs.

Is there any workaround for this problem?