Why do I get an error 'Cannot resolve symbol <symbolname>' in ReSharper?

Question

In Resharper you can disable inspection for that file with: Ctrl + Shift + Alt + 8.

So no final resolution yet. According to tech-support this is a known issue and is being researched. Final resolution undetermined at this time. If you're experiencing a similar issue, just hang tight. URL: http://youtrack.jetbrains.net/issue/RSRP-178681

Nothing worked for me until I followed this answer -- removing the related reference from the Project/References and then adding it again.

This worked in my case (Visual Studio 2015): Go to Menu and select Options. From the opened window, select ReSharper Ultimate (yours might be different edition) and click Suspend. That will basically suspend all the resharper features. Then click "Resume"

That should work. Otherwise, try to clean Resharper cache.

I just tried something odd and it worked. In your page directive, change the AutoEventWireup to false, SAVE the file, then change it back to true. It's some connection between the aspx file and the designer I think.