This usually happens in the _LoginPartial.cshtml or _ManageNav.cshtml razor view. Eg.
@inject UserManager<IdentityUser> userManager
Must be changed to
@inject UserManager<MyUserStore> userManager
The same applies to
When registering your own
MyUserStore (bad name, should be MyUser) for the AspNetCore Identity, the UserManager<> type will be registered to the ServiceCollection as
Whenever you want to resolve the
UserManager<>, specify the identity user model registered in your startup as the type parameter. Which would be
UserManager<MyUserStore> in your specific case:
Or like-wise, when resolving it inside other classes, as may be the case in your
Seeder service. The call stack of your exception should give you a hint of where this is happening.