I have the Receiver 7255: I blocked some channels but I have now forgotten the password.
On page 63 of the English section of the User Manual, it says that when you forget the password you can select the factory default option.
When I tried the factory default option it still prompted me for a password.
I have tried all the available passwords from 0001 to 9999, unfortunately nothing works.
I think it is something to do with the software.
On page 41 of your Manual, as you can read below, there is a special code (1250) in case you forget your password.
Another option is to set the receiver again to the factory defaults, but you will lose your personal settings.