mod_rewrite problem

[cerebus]

I want to redirect f1.html to forumdisplay.php?f=1 to follow the old pattern of a forum. This is done on a separate development server with vbulletin 3.5.1

RewriteRule ^f([0-9]+)\.html$ forumdisplay.php?f=$1 [L]
Gives me an empty page. Log shows a 200 found and 0 byte file.

RewriteRule ^f([0-9]+)\.html$ testfile.php?f=$1 [L]
testfile.php being a test page that only show $_GET[f] and this works just fine.

If I do a 301 redirect with [R], the redirect to forumdisplay.php also works.

Anyone have any idea why the first rewrite doesn't work? I have searched for people having the same problem, but not found anything helpful.