I had a similiar issue.  Mine occured on content pages where I had not defined the master page at the top.  I was setting my master page in the web.config and didn't want it on each page.  I wonder if there is a way to make those errors go away.  The site still runs though.

~ Lisa

Wow, this is a bizzare one. Same situation, using master pages, and when I try and edit a file without the master page open, visual studio pronounces that all of my <asp: tags are invalid :-/

Oh well, a workaround is a workaround I suppose.

Thanks again ASP.Net forums / google

This bug is still occurring in vs 2005 service pack 1.

In my case opening master pages didnt help. But opening web.config and making a whitespace change to it, then clean solution and rebuild, did make the problem go away. None of my master pages are present in web.config, in fact I've never made a change to web.config

plq1:

This bug is still occurring in vs 2005 service pack 1.

In my case opening master pages didnt help. But opening web.config and making a whitespace change to it, then clean solution and rebuild, did make the problem go away. None of my master pages are present in web.config, in fact I've never made a change to web.config

What is 'clean solution'?  I'm having this same problem, even with my master page and web.config files open.  I just upgraded my AJAX and Toolkit to the release versions.  I didn't have any errors/warnings on the previous versions.

Please read carefully Ajax RTM release information. You must remove old assemblies from bin as RTM version installs in GAC, not in bin. You also need to fix web.config.

http://ajax.asp.net/documentation/Migration_Guide_CTP_to_RTM.aspx 

http://ajax.asp.net/documentation/Migration_Guide_RC_to_RTM.aspx

http://ajax.asp.net/documentation/AspNet_AJAX_CTP_to_RTM_Whitepaper.aspx 

Changing web.config and saving fixed it for me as well.  Thanks guys!