Hi Simon,
Thanks for your sample. I met the exception when run it.
The "Unspecified error" thrown by Openlayers. It's this JS code "supported:function(){return!!document.namespaces}". Looks document.namespaces have the unspecified error.
I add your code to a new project I build, in same project my page run but your page met many different errors after I did some change.
I think that should related your page, it looks your page have some different with mine in the aspx file and the aspx.designer.cs file. I am not sure how your create your page but I think maybe you can did some test also.
I have attached my test project in this post.
The most strange thing is if I change your page, sometimes the same code will have different result, one time it throw exception, after I did some change then change back, the other time it run correct. I have clean cache, close port and rebuild each time.
Sorry I did some changes in your files for test in this project, you can replace it with your original files.
I think that's shouldn't the problem of the map control.
Regards,
Don
001_ThinkgeoWebSample.zip (147 KB)