Unable to access Plesk after upgrade to 12.5.30: Upgrade stuck and login page is blank

Applies to:
Plesk 12.5 for Linux
Plesk 12.0 for Linux

Symptoms

Plesk GUI is inaccessible with error 504 gateway timeout (nginx)

OR

Plesk upgrade/update stuck at this stage for a long time:

===> Plesk database scheme upgrade has been completed

OR

===> Cumulative upgrade of Plesk (final stage) has been completed.

In ps auxf utility output it is possible to find this string or something similar that is not changing for a long time:

_ /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/admin/plib/Upgrade/upgrade.php

After killing this process, upgrade can be continued or it hangs again.

It may be not possible to login to Plesk - login page is blank.

Running Plesk autoinstaller again may result in halt at this place:

gpg-pubkey-c105b9de-4e0fd3a3 gpg(CentOS-6 Key (CentOS 6 Official Signing Key) <c
gpg-pubkey-914bdf7e-55c05220 gpg(Plesk Team <info@plesk.com>)

OR

Ign http://autoinstall.plesk.com all/all Translation-en
Ign http://autoinstall.plesk.com trusty/all Translation-en
Fetched 221 kB in 3s (57.1 kB/s)
Reading package lists...
ERROR: Interrupted by user
Reading package lists...

Command plesk repair installation and other Plesk related commands may also stuck.

This is a software issue with internal ID PPPM-3717. The fix will be included in further Plesk updates.

As workaround, remove lock_manager temporary files:

rm -f /usr/local/psa/var/cache/SharedLockManagerStorage*

Restart services:

service sw-engine restart && service sw-cp-server restart

Run Plesk installer and continue upgrade. Execute plesk repair installation if it is required.