Unable to access shared folders

Hi All, I'v got 2 (old) identical readynas nv+ v1 devices. I just configured the second one , exactly the same as the first one. But every time I want to access one of the shared folders it's asking me for a user name and password. (Header from pop up window: "Windows security, enter your credentials to connect to:NAS...) The pop up window looks like a NAS window, not similar to a general windows pop up. Both the user name and password I don't know. Tried , my windows 10 password and the admin password from the nas, which both are not working. Both the devices are running on Raidiator 4.1.14( I even downgraded the problematic device) What I did so far: 1 Restart computer and NAS (several times) 2 Compared the settings to be identical on both devices(100%, unless I overlook some submenu) 3 removed entries for the nas in windows credential manager 4 Turned of password protected sharing in Windows 10 5 searched on google the last 4 hours to find a solution....... Hope that somebody can help.....