As you may know, FileMaker WebDirect guide describe many unsupported FileMaker functions in WebDirect. In page #9 of this guide, you will find this paragraph :
• There are some things you can do in FileMaker Pro that you should avoid in your FileMaker WebDirect solution.
Some features, like opening multiple windows, don’t translate well to the web. Other features, like rich text, simply don’t work. Consider designing a new solution specifically for
FileMaker WebDirect, or extending the portions of your existing solution that make the most sense on the web. See “Step 2: Understand the capabilities of FileMaker WebDirect” on page 13.
I realize that doesn't help you but i encourage you to read carefully this entire guide in order to avoid other bad surprises...