Thanks for the reply Kenneth.

The control does work on other pages within the same site. We are using IIS 7 in classic mode.

Further progress has been made, though. On the page load, the control is inside a runat="Server" div which is set to visible="false" on load.

When it does load, then we get this error.

If the control is visible from the start it is ok.

I'm just testing locally now with the latest version of the dll to see if I can replicate that.

The server is 2008.