Response status code was unacceptable 500 error usually appears when you are unable to connect to Atlas VPN servers. The error can be resolved by changing the connection protocol. To do that, follow these steps:
Windows and macOS
Open the Atlas VPN app, click on Settings, navigate to the Protocols tab, and choose one of the available protocols.
Android and iOS
Tap the cogwheel button at the bottom left side of the Atlas VPN app, and go to the VPN protocol.
Android TV and Fire TV
In the Atlas VPN app, press the Settings button on the left side menu, and go to the Settings section.
Press the Protocol button to access the protocol menu.
Lastly, choose your preferred connection protocol. And that is it!