Unable to access Backup Manager in Plesk: 504 Gateway Timeout

Applicable to:

Unable to access Plesk > Tools & Settings > Backup Manager, the following error is shown in browser:
CONFIG_TEXT: 504 Gateway Timeout
There are the following records in /var/log/sw-cp-server/error_log file:
CONFIG_TEXT: [error] 3235#0: *1483 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 123.123.123.12, server: , request: "GET /plesk/server/backup/local-repository/ HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock", host: "11.12.13.14:8443", referrer: "https://11.12.13.14:8443/admin/server/tools?context=tools"
It is possible to find such system process while trying to access Backup Manager:
# ps ax | grep pmm
11357 ? R 0:37 /usr/bin/python /usr/local/psa/admin/sbin/pmmcli --get-tasks-list Restore,Backup 5dfb2ce3-1743-4cac-902e-78773bc245d6 any
There are a lot of "Permission denied" messages in /usr/local/psa/PMM/sessions/TIME_STAMP/migration.result files:
# egrep -lir 'Permission denied' /usr/local/psa/PMM/sessions/ | xargs wc -l
34544 /usr/local/psa/PMM/sessions/2013-11-13-030003.384/migration.result
34544 /usr/local/psa/PMM/sessions/2013-12-22-013005.54/migration.result
34543 /usr/local/psa/PMM/sessions/2013-12-05-013004.227/migration.result
34534 /usr/local/psa/PMM/sessions/2013-11-06-030003.347/migration.result
35502 /usr/local/psa/PMM/sessions/2013-11-10-030003.623/migration.result

Product issue:

#PPPM-7211 "Support issue #PPPM-7211"
Fixed in:
- Plesk Onyx 17.8 17 April 2018

Please consider updating your server:

If update is not possible for some reason you may try the following

temporary solution

Upgrade Plesk to the latest release:

If the upgrade is not possible, as a workaround do the following:

Find out the files with incorrect permissions. Backup manager was not able to get the access to content mentioned in migration.result files to place it to archive. Use the instructions from the article Plesk backup task is failing: Cannot stat: Permission denied.
Backup the current sessions directory before applying any changes:
1. Log in the server via SSH.
2. Find out the running pmmcli processes:
  # mkdir /usr/local/psa/PMM/sessions_backup/
  
  # cp -a /usr/local/psa/PMM/sessions/* /usr/local/psa/PMM/sessions_backup/
Remove the the lines with 'Permission denied' message from /usr/local/psa/PMM/sessions/*/migration.result files:

# grep -lir 'Permission denied' /usr/local/psa/PMM/sessions/ | xargs sed -i '{;/denied/d;}'
Find out the running pmmcli processes:

# ps ax | grep pmmcli | grep -v grep
12146 ? R 49:39 /usr/bin/python /usr/local/psa/admin/sbin/pmmcli --get-tasks-list Restore,Backup 73406802-7e5a-44e4-9b14-aa2cbf201973 any
13275 ? R 36:15 /usr/bin/python /usr/local/psa/admin/sbin/pmmcli --get-tasks-list Restore,Backup 73406802-7e5a-44e4-9b14-aa2cbf201973 any
Kill the current pmmcli processes if they do not refer to current migrations and backup processes:

# kill -9 12146 13275
Check the access to Backup Manager.