If Quantower was connected to Rithmic before via the RTrader Plugin (i.e. the checkbox Use RTrader is active), and at the moment the RTrader Pro platform is not connected or the Allow Plugins option is not active there, then Quantower will not be able to connect.
Solution (several different options, not step-by-step):
1) start RTrader Pro platform with Allow Plugins mode and connect again to your account in Quantower
2) try to connect to Rithmic as a direct connection: close RTrader Pro platform, disable Use Rtrader option in Quantower settings and restart Quantower. After start Quantower again and connect to your account as a direct connection