You are probably running into an IE limitation in the length of URLs. See this thread for more info.
There are no easy options. However, you can use a POST call instead of the default GET. (I believe there was a discussion here on BOB about this, but I’m not able to find it.) What you need to do is embed some Javascript code in the calling report. The code will construct an HTML form, and make the call to openDocument using the POST method. Since POST parameters don’t have the same length limitation, they can be an indefinite length.
Joe
joepeters 
(BOB member since 2002-08-29)