Cannot access Plesk-hosted website on iOS or Mac devices: The operation couldn’t be completed. Protocol error

Symptoms

• On iOS/MacOS devices it is not possible to load website in any browser with an error like below:

  CONFIG_TEXT: The operation couldn’t be completed. Protocol error

  ---

  Blank page may also be shown 
• Accessing website from Debian/Ubuntu command line returns an error similar to below one:

  # curl -vvv -k -L -I example.com
  * http2 error: Invalid HTTP header field was received: frame type: 1, stream: 1, name: [upgrade], value: [h2,h2c]
  * HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)
  * stopped the pause stream!
  * Connection #1 to host example.com left intact
  curl: (92) HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)

• nginx set up as proxy server in Domains > example.com > Apache & nginx settings > Proxy Mode;

Cause

Issue is caused by Apache bug 59311: when http2 module is enabled both for Apache and nginx, Apache sends incorrect header to nginx. When nginx used as proxy, it passes this invalid header further to browsers causing them refuse to connect. 

Resolution

To fix the issue:

1. Login into Plesk;
2. Disable http2 for Apache in Tools & Settings > Apache Web Server.