Hi Jens,
thank you for all the clarifications.
I performed two tests to be sure, once again, that is a problem due to the last upgrade consequences.
The first is the one you suggested above resetting through CTRL-Amiga-Amiga and the DisMo button, and yes I confirm that without loading/displaying the menu the df0 works 100% (along with "click"sound).
The second was a bit raw and risky test, in fact I opened my Amiga case, I removed the upper part of the case. I turned on Amiga and selected from ACA500Plus menu a choice (eg. Kickstart 1.3). After that screen returned to classic hand with floppy (and in my case along with the df0 unit not working), I unplugged the power connector of floppy unit from Amiga motherboard for 1 second. When I plugged back it then df0: returned to work.
It seems that something in the firmware of ACA500Plus paralizes or is in conflict with Amiga's controller of df0 unit.
In any case, looking at the result of the test you suggested (CTRL-Amiga-Amiga and the DisMo), what is the best way to solve the problem regarding the menu? Because at this point I assume that the problem arises from a bug or a bad upgrading of the firmware, right?
Thank you again for your precious support. I hope that this topic can be useful to other users as well.