As Mikhal Drozdovich noted in his comments, the solution is to check the SQLServerCe component references in all projects. All of them should point to the same version, which should correspond to the latest installed version on the computer from which you are debugging. Therefore, fixing should be as simple as replacing all assembly references with a reference to
%YourProgramFilesDir%\Microsoft SQL Server Compact Edition\v3.5\Devices\System.Data.SqlServerCe.dll
(The place where you installed SQLServerCe.)
If this is fixed, you may also need to install previously automatically deployed cabin files manually.
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