thanks for your replies. So far I am still not lucky:
4 ERR:303-No serial ports found.
3 Path to EEPROM: "C:\Users\AstroLuke\Documents\PMC8Firmware\9t_17a_EXOS2.eeprom"
2 Path to Propellent.exe:"C:\Program Files (x86)\Explore Scientific Utilities\PMC-8 Configuration Manager 1.0.3\Propellent.exe "
1 Attempting to upload ROM
0 Mount appears to be connected on Serial--
I'm already using v1.03 of the configuration manager. I tried rebooting PC aswell as PMC eigth and reconnected the serial cables as suggested. In general the connection via serial works great, no problems when connecting and driving the mount.
In the configuration manager on tab "connection" is a button "Find" next to COM Ports. If I press it, it says no com-ports found. But on the other hand it directly shows the correct com-port in this drop down window. I only have that one com port. it is COM3. If I plug the usb-serial cable in a different usb port it switches to COM4 or COM5. The configuration manager shows the correct com-port every time, but at the same time it complains, that it can't find any com-port. I am confused...
Mounts: ES PMC-8 EXOS-2
Scopes: ES AR152 refractor, Tamron zoom-lens 70-300 mm
Cameras: Sony a6000
Software: Siril, Affinity Photo, Sony Remote Control