This is so weired and so far I haven't found any infromation.

I have a form library with web-enabled template and it works fine. I
had some customized workflow send out emails with link to this
library and all works OK except workflow will eat out some space of
URL. but this is not the point.

I was testing with an out-of-box approval workflow and it will send
out the URL point to the form like this:

http://portal2007/sites/sitename/_layouts/formservices.aspx?
libraryname/formname.xml

When I use this link, sometimes I received an error message telling
me that this form has an unsupported relative URL. Sometimes it only
show "Yhis form has been closed" without any prompt.

Well, it's MOSS created and email this URL, how can I change it or
make it works?