Jens,
I'm a little bit confused here. var Global = xdc.useModule('ti.ndk.config.Global'); was already in the client.cfg when it worked? What was added to the client.cfg example? Can you repost the broken client.cfg?
On another note, NDK 2.21 doesn't support DSP/BIOS either, so I don't know if the DSP/BIOS wrappers are causing some issues here. You can perhaps try NDK 2.20, but I don't know if these products will work with it.
MCSDK 2.1.2.6
MCSDK PDK TMS320C6657
Where did you get the client application from? Was it from some SDK?