Somehow I managed to get the F5OEO firmware (version August 29th, 2020) to work and maintain the frequency extension active.
What I basically did was to makes sure I had the original ADALM PLUTO firmware 0.31 (not 0.32!) on the device and performed the frequency extension.
After that I loaded the pluto.frm from F5OEO into the Pluto folder (Windows 10) and started the uploading process. Firmware is functioning, frequency extension was tested with SATSAGEN.
I have no idea why it didn't do this the last time I tried it, but now it works (well, I understand it's the digital world: either things work -- or thry don't).
Holger 'Geri', DK8KW