Upgrade from 11.0.9 to 12.5.30

[snookersky]

I am encountering a problem while upgrading Plesk. None of the two target versions proposed are working

For 12.0.18 (Stable) following error occurs:

ERROR while trying to execute SQL query, the query was: INSERT INTO `ServiceNodeConfiguration` (`serviceNodeId`, `section`, `name`, `value`) SELECT '1', 'webProxy', 'enabled', `value` FROM `ServiceNodeProperties` WHERE `serviceNodeId` = '1' AND `name` = 'server.nginx.enabled'

Investigating the logfile, I could see:

===> Preparing Plesk database upgrade (revertable stage).
ERROR 1062 (23000) at line 1: Duplicate entry '1-webProxy-enabled' for key 'PRIMARY'
InnoDB

For 12.5.30 another error is showing:

Launching component checkers...
File downloading PSA_12.5.30/examiners/panel_preupgrade_checker.php: 21%..36%..47%..58%..70%..81%..97%..100% was finished.
File downloading PSA_12.5.30/examiners/php_launcher.sh: 100% was finished.

Components validation detected at least one important issue:
Parallels Panel pre-upgrade check...
  PHP Parse error:  syntax error, unexpected '[' in /root/parallels/PSA_12.5.30/examiners/panel_preupgrade_checker.php on line 2168

It seems that the file 'panel_preupgrade_checker.php' just downloaded has a syntax error...

It's true, I am upgrading raraely, but the reason for that is exactly the issues I am encountring every time (I am using Plesk since version 6). It's better these days, but when it'll be possible to upgrade w/o any problem?