Unable to upload Plesk license: 502 bad gateway

Symptoms

• Unable to apply the Plesk license to the server

• The following message is shown in Plesk UI when trying to upload a license key file:

  PLESK_INFO: 502 Bad gateway

• The following messages are present in /var/log/sw-cp-server/error_log:

  [error] 20770#0: *272 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 91.204.25.4, server: , request: "POST /server/key_upload.php HTTP/1.1", upstream: fastcgi://unix:/var/run/sw-engine.sock:", host: "52.53.62.41:8443", referrer: "https://52.53.62.41:8443/server/key_upload.php"

• The following messages are present in /var/log/sw-cp-server/sw-engine.log:

  WARNING: [pool plesk] child 2268 exited with code 127 after 11.528033 seconds from start
   NOTICE: [pool plesk] child 2283 started

Cause

The opcache module is not disabled for Plesk engine service (sw-engine).

Resolution

1. Connect to the server via SSH

2. Edit the file /usr/local/psa/admin/conf/php.ini with text editor. Make sure that opcache is disabled. The relevant line should look like:

   opcache.enabled = 0

3. Restart sw-cp-server and sw-engine:

   # service sw-cp-server restart
   # service sw-engine restart