Superuser lost?

Hi,

I have Synology NAS box at home with RAID array that uses Postgres to store
various system settings.
Some days ago I had troubles with my array so I did some manual assembly
after which my NAS needed to perform migration. So I did that.
After the migration I noticed some apps didn't work properly and after some
digging I found /var/log/postgres.log being spammed with "FATAL: role
"postgres" does not exist".
Trying to find out what exactly is wrong since "psql -U postgres" gives me
the same error.
I tried all the usual logins (root, admin, mediaserver) and all ends with
same message.
User postgres is correctly defined in /etc/passwd.

Is it possible that the user has been "somehow" deleted during migration?
And if so is there any other way how to fix this? (createuser obviously
doesn't work..).
Maybe do some "reinitialization" ?

I know I could just reinstall postgres but I would like to avoid this
since I don't know the possible impact on whole system.

Thanks for any pointers!