Hi Geir,
Are you updating your applicatipon from any older versions? If yes, then
please try using this utility to convert the resex files.
You can access the conversion tool by the following.
Start->ProgramFiles->Syncfusion->EssentialStudio 3.3.0.0->Utilities->Migration-> "Now choose the version".
Here is a KB showing how to run the tool.
http://www.syncfusion.com/Support/article.aspx?id=10370
You can manually open the .resx file (in the VS.NET editor) and check you have only current 3.3 version.
If this doesnot solves the issue,please send us a small sample with the issue. Also please let us know the VS.NET version.
Best Regards
Mouli.