Thanks Steffen, that helped instantly!
FYI as this might help others too:
I was not able to upgrade from UC SW 3.3.x to 4.x via RPRM, that was only possible through the FTP provisioning server (yes, the bootrom files were in the resource folder in RPRM)
Once the device was on 4.x, I could easily up- and downgrade within the 4.x releases via the RPRM though.
Best regards
