Just throwing my two cents in...
Within my dbConfiguration.cs, I like to wrap my context.SaveChanges() method into a try/catch and produce an output text file that allows me to read the Error(s) clearly, and this code also timestamps them - handy if you run into more than one error at different times!
try
{
context.SaveChanges();
}
catch (DbEntityValidationException e)
{
//Create empty list to capture Validation error(s)
var outputLines = new List<string>();
foreach (var eve in e.EntityValidationErrors)
{
outputLines.Add(
$"{DateTime.Now}: Entity of type \"{eve.Entry.Entity.GetType().Name}\" in state \"{eve.Entry.State}\" has the following validation errors:");
outputLines.AddRange(eve.ValidationErrors.Select(ve =>
$"- Property: \"{ve.PropertyName}\", Error: \"{ve.ErrorMessage}\""));
}
//Write to external file
File.AppendAllLines(@"c:\temp\dbErrors.txt", outputLines);
throw;
}