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 SignInManager.
When registering your own MyUserStore (bad name, should be MyUser) for the AspNetCore Identity, the UserManager<> type will be registered to the ServiceCollection as UserManager<MyUserStore>.
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.