As Brad Wilson states in his answer here:
ModelState.IsValid tells you if any model errors have been added to ModelState.
The default model binder will add some errors for basic type conversion issues (for example, passing a non-number for something which is an "int"). You can populate ModelState more fully based on whatever validation system you're using.
Try using :-
if (!ModelState.IsValid)
{
var errors = ModelState.SelectMany(x => x.Value.Errors.Select(z => z.Exception));
// Breakpoint, Log or examine the list with Exceptions.
}
If it helps catching you the error. Courtesy this and this