WordPress file manager error – Unable to connect to backend. HTTP error 500
PS- screenshot is captured at the time of error
cause – the issue is because you have wordpress plugin conflict , youo can verify it by desibling all plugins or follow https://powershelltalk.com/check-wordpress-plugin-theme-conflicts/
Solution- in my case the conflict was with file manager and Ninja forms, so i disabled ninja form and everything is working fine now.