Unknown problems with Dataset in .NET

DataSet object generated by Wizard started to show Exception while opening it in Designer:
key cannot be used in specified...


Also it is impossible to create new Dataset with TableAdapters..

The only solution found:

remove folders Visual Studio in these folders:
1. c:\Documents and Settings\%CurrentUser%\Application Data\Microsoft\
2. c:\Documents and Settings\%CurrentUser%\Local Settings\Application Data\Microsoft\