This could be caused by the issue of Host systems that have changed there IP Address because of the DHCP server. If you can change the IP Address on the host system to the IP Address from the allowed list this would be easier. If not then you will have to go to the console screen and enter ctrl. + alt. + x for Extended tools menu and select Restore Default Administrator Settings.
To allow all host systems to access the GUI on the server via HTTPS leave the field Administrator access empty.
Below is additional information on this function.
Function: “Administrator access”
The administrator can precisely assign IP address (separated by a semicolon) that is granted authorization to access to Open-E NAS Web administration. Please exercise caution with this function, if all computers in the network receive IP addresses via DHCP: Today’s IP can be replaced by a new one after the lease ends.
Open-E NAS can assign a total of nine IP address, meaning that you should enter all nine when in doubt (192.168.0.0 to 192.168.0.8.)