InfoPath Dev

I had to reply to this because I had the same error and did not come across a solution until I took some steps....

Running Windows 7 with Office 2007 and VS 2010.

I was cruising right along developing some code-behind for various InfoPath forms and then one day I could no longer open the code behind.
I could right-click and enter Design mode - no issue.  But clicking the menus to open the code-behind resulted in the above error.

Clicking the various buttons (which had the code behind and would normally open VSTA) resulted in the above error.

I checked and dug in group policy (I was aware of the CustomCode = Disallow).
I ran procmon and started checking the "missing" registry keys. 
I checked event logs, error logs, and anything else I could get my hands on - nothing helped.

Finally I was able to fix the form AND replicate how I fixed it.

1. Open the form...
2. Publish the form...
3. Cancel publishing when you chose where to deploy the form...

Now you'll be able to edit the code behind.

I have replicated this over and over so I'm not sure what Publishing does to "reconnect" the code-behind that simply opening the code-behind doesn't - but here is a possible solution for the above error

I am having this error for a form stored on my desktop.  I can open the form in IP Filler 2010, but when I right-click and select Design, it gives me this very generic error and won't open.  I made some edits to it last night, but nothing from a code perspective - only receiving some Sharepoint list data into some drop-down fields.  Anyone have any idea how to open this file in Design mode?  I've tried relabeling it to a .cab file, extracting the source files, and opening the manifest.xsf file, but I get the same error.

I'm rather new to IP 2010 so take it easy on me. FYI, I copied this file from a Sharepoint form library, and have made some local edits to it.  I'll publish it to a different form library when complete.