nRF52832 Serial DFU - Transfer Init Packet - No Response after "Execute Command"


I am currently implementing the protocol for a serial DFU, following the instructions given here:

The general flow I implemented is:

Get MTU (0x07) --> Select Command Object (0x06 0x01) --> Create Command Object (0x01 0x01)  --> Write up to MTU bytes of data (0x08 XX .... XX) --> Execute Command (0x04)

Up to "Execute Command" everything seems to work perfectly fine. The problem is, I do not get any response on the execute command - neither a "Response Execute Success" nor any failure. 

Do you have any idea, what I am doing wrong here?