Yes, this issue is indeed strange, a possible cause could be if the administrators role id in your SecurityConfig.config match another role (e.g. BackendUsers) in your database - you can find this in table [sf_roles]
You can try create an administrator user using the following script:
var userManager = UserManager.GetManager(
var user = userManager.CreateUser(
RoleManager roleManager = RoleManager.GetManager(
var role = roleManager.GetRole(
and see if it logs successfully. If this user succeed to log as admin, then you can re-assign the rest of your admin users to "Administrators" role.
Hope this helps.
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