"Operation could not be completed."
There are a few things you can do to try and fix it:
- Remove all the the local cache files from your machine (typically found in C:\Users\USERNAME\AppData\Local\Microsoft\InfoPath) - delete the "Designer2", "Cache2" and temp folders
- If that doesn't work, try running the form (e.g. in Visual Studio 2008, just right click on the form project and Click "Debug -> Start New Instance". When it opens, it will correct the project structure or re-add neccessary .NET assemblies to the project. Go to design mode again - and it will all start working (though all the tabs in the Designer window will now open). I suspect this happens if the xsf file is not correctly written to or some of your supporting files are readonly on save.