Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 29.0.5): 30.0.1
Operating system and version (eg, Ubuntu 24.04): Synology NAS
The issue you are facing:
I was trying to link Authentik OpenID using username attribute match option and probably set something incorrectly. I am able to login to nextcloud using authentik openID, but none of the users now have Admin privileges. I have earlier disabled default Admin account and now even that seems to be gone. Effectively my nextcloud install is not having any Admin user. How do I re-instate an admin user plz ?
Is this the first time you’ve seen this error? (Y/N): Y
The output of your Nextcloud log in Admin > Logging:
Unable to Login as Admin
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
PASTE HERE
Output errors in nextcloud.log in /var/www/ or as admin user in top right menu, filtering for errors. Use a pastebin service if necessary.
PASTE HERE