This happened to me whilst I was trying to debug my WebPart from Visual Studio, always after the 'first' debug session it failed to re-load and thus, threw
$Resources:core,ImportErrorMessage; when launching the 2nd debug session.
Finally I found what worked for me was:
Visual Studio -> Build -> Select 'Retract'
Upon doing so, I was able to Clean the solution, re-deploy and then finally start debugging as normal.
I know it's quite a small thing, but other resources on the web was also suggesting to:
- Add Guid Attribute to the WebPart (To make sure there's no mis-match)
- Rename the WebPart to .wsp
- Restart Visual Studio and the debugging browser all together (With Admin rights too!)