Hey guys,
first of all great thanks for your hints.
After nearly getting crazy with this, I made it to update my POD HD500. What worked for me was to set the POD to flash mode as described above, connect it to Monkey and run the flash install. But I had to start the update several times. With every new start more of the firmware was written to the device until the update was completed. It took me 4 starts to end up successfully. Now also the HD Edit can connect to the device again.
What a mess... :-)
Greetings
JJ