A couple things can cause that error.
First, FTM uses Internet Explorer regardless of what else is on your system. So first thing to do is try opening that public member page in Internet Explorer directly. If you still get the script error, it's not an FTM issue - it's an IE issue. In that case, try clearing IE cookies & temporary internet files and see if that helps:http://ftm.custhelp.com/app/answers/detail/a_id/483
If you don't get the error in IE but still do in FTM, or if you get it in both & clearing temp files doesn't help, most likely cause is security software mucking with the data coming from ancestry.com's web servers. Disable or reconfigure security software.
Long shot is ISP filtering, you can rule that out by trying with IE on a different computer using the same Internet connection.