I need to make several 301 URLs since the old website didn't use Joomla but had a lot of inboud links.
My problem is that the 301 tool seems not to work properly with this kind of URL:
display.php?categoria=1&guida=A%20PIEDI%20NEL%20LAZIO
it works fine with regular URLs without %A20 but it fails with this particular pattern.
Any idea how to solve this?

%20 is a URL encoded value of space " " character.
So did you try to put there the URL using spaces, not the already encoded characters?

no, exactly the url with the encoded spaces, as you read it in the previous message

Then I think you should try that.

nothing.
It didn't work.
It gives me a 404 page
:-|