Two tips to those who still have problems downgrading:

1. Open the embedded web server (enter the IP address of your printer in a web browser). Go to Settings > Security > Administrator Settings. In the "Printer Updates" section, make sure that the "All Firmware Updates" checkbox is checked and the "Only Web-Downloaded Firmware Updates" checkbox is unchecked.

2. Instead of using a USB stick, there is also the option of sending the rfu file to the printer via LPR (this works because rfu files are simply formatted as raw print jobs, which the printer understands as firmware updates). To do this, first activate "LPR print services" in the Windows functions ("Switch Windows functions on and off"), which installs the command line program lpr.exe. Then you simply need to execute:

lpr -S [printeripaddress] -P [printeripaddress] [filename.rfu]