ReadyNAS 104 asking me to 'Authenticate' before drag and drop

I don’t use my ReadyNAS 104 very much but tonight I ran into a problem I hadn’t come across before. I was trying to drag and drop some files from one of my Macs to my ReadyNAS. I’m sure I’ve done this many times without any issue. But tonight when I tried I got the error message ‘Modifying “my NAS name” requires an administrator name and password. To move ‘the file in question’ click Authenticate”.

I’ve never seen this message before and there is no option to ‘Authenticate’. There is just an ‘OK’ within the error message box. As I said I don’t use my NAS a lot so wondered if maybe a recent update had changed something. I’m running the latest version firmware.

My Mac’s Finder initially shows me connected as ‘Guest’. I tried connecting with a ‘Username’ and ‘Password’ that I’d created when I first got my NAS. To be honest I’ve no idea why I had to create that, and I’m very much a newbie to NAS drives. But it did allow me to connect to my NAS with that identity and it created a new folder with that identity name on my ReadyNAS. I was able to transfer files to that new folder but only to that folder. Anything else produced the error message referred to above.  My NAS appears to be fine in other ways - I can open folders and files, and the Media Player is working fine for music files.  I'm running OS X 10.10.5 on my Macs.

I’m certain that before tonight I was able to drag and drop any files I wanted to to my NAS without any error message or request for me to ‘Authenticate’. I’m not aware of any system changes that might explain this new behaviour. Can anyone help and maybe let me know what might be going on?

Thanks!