You're receiving the error message "Your browser does not seem to be accepting cookies." when attempting to log in and the "Password" field is disabled:

This occurs when setting the "IsHttpsEnabled" option to True without having actually enabled HTTPS in theappsettings.json file, usually copied into the %ProgramData%\Red Gate\SQL Monitor folder, but also possibly the one in the installation directory (C:\Program Files\Red Gate\SQL Monitor\Web by default). 

To fix the problem revert the file to the default and restart the web service (or site and application pool in IIS).  You should then enable HTTPS following these instructions if using the Kestrel webserver binding a port and certificate in IIS.  Once this is done the "IsHttpsEnabled" setting will automatically change to True and the other related settings will reflect that HTTPS is enabled.