RDP Internal Error

Hi , there can be multiple reason, if CloudWatch alarms configured, check if any alarms of cpu/memory or disk, also as per link suggested Indranil, please check telnet is working and also check system log and instance screenshots of your instance status from Monitoring & troubleshooting under Action drop down.

Lastly , if you still has no clue, troubleshoot with serial console. https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/ec2-serial-console.html

Can you please take a look at this troubleshooting guide to see if it helps - https://aws.amazon.com/premiumsupport/knowledge-center/troubleshoot-remote-desktop-connection-ec2-windows/