For those who face this error and cannot connect to public or personal servers, I have a solution for you.
Follow the exact steps listed below:
1. Restart the PC or be sure that you closed every instance of GTA V & Rage MP.
2. Go to Documents(usually located in your Root disk, where the operating system is installed), find 'Rockstar Games', open it and delete the 'Social Club' folder.
3. Open updater.exe(located in the Rage MP folder) and connect to a server.
4. Rage will open the Rockstar Games Launcher and will ask you to connect to your Rockstar Games account, so do that.
5. After that, you'll notice that Rage MP has stopped, open Rage MP again and connect to a server.
It took me a few days to solve this problem, leave a reply if it helped you somehow.
- STKappa