Hi all,
Me too I thought I could give out my solution to a similar problem ; hopefully it will help.
Long story made short :
On a VB.NET 3.5 + AJAX web site, I wanted my visitors to be able to navigate inside one page unsing anchors.
Anchors inside content rendered by a listview inside an update panel are ok, but....believe it or not, the "top" anchor, referring to the top of page was causing the "unknown error" !
Anchor was <a name="top" /and was located outside the update panel, under the opening form tag and before the scriptmanager tag.
Got rid of it and all worked fine again. Don't ask why...
Happy programming.
JH