The easiest solution is to create an admin user trough code. You can use the following code snippet:
var userManager = UserManager.GetManager(
var user = userManager.CreateUser(
RoleManager roleManager = RoleManager.GetManager(
var role = roleManager.GetRole(
Try to log in using the newly created user. If the message for maximum concurrent users appears, you can click "Force someone to logout" and logut all users you need.
Hope this helps.
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