ReadyNAS Ultra 4 default password fails after OS reinstall

After failing to logon to my ReadyNAS Ultra 4 as 'admin' using my regular password, I did a password recovery by entering my email address and answering the recovery question. It came back saying it had sent a new password to the address I had entered. Unfortunately, I never recieved any password, not after waiting more than 48 hours, checking all inboxes I could think of. 

So, I did a OS reinstall as instructed elsewhere in this community. According to the display of my unit, the firmware was updated during the process. However, the default ID/password combination ('admin'/'netgear1') does not work. Whatever computer or browser I use. 

Me and my family have been using this NAS for 6 years now, so you can imagine that back to Factory Defaults is not an option. 

Does anyone have a clue how I can solve this?