When calling DbContext.SaveChanges() you receive the error message
Error 1012 (Provider v. 18.104.22.1687): Connection must be opened \r\nLine #: 0; Column #: 0
The Entity Framework provider for VistaDB has generated an invalid SQL statement in EF 6 when querying rows added to tables that use an autogenerated primary key (e.g. @@Identity)
This was fixed in the VistaDB Provider for Entity Framework 6 version 22.214.171.1240. Update from NuGet to be sure you're using this version or later.