Typical reasons for an upgrade to fail :

You need to upgrade the versions in order they were released i.e you cannot upgrade from a v5.1 to v6.5 directly.
You need to first do a v5.1 to v6.0 and then v6.5 in the same order.
The Upgrade URL you are entering is incorrect. Verify that it is http://server_name:8888/psa/site.Upgrade.do?
Typing anything else, will give you an error.
You have copied the license.properties, site.properties, db.properties from the old installation directory to the new directory.
The upgrade process that you run will copy the 3 files to the new directory. Hence, you are asked for your current installation directory. DO NOT copy these 3 files manually.
During the upgrade process, you are prompted to enter the current installation directory. Please enter the correct path.
Your Upgrade has failed once and you are restarting the Upgrade process without deleting the psa_x.y.z directory where the upgrade failed.
You need to delete the directory and unzip it again before starting the upgrade.
Make sure that your existing installation is working fine before you proceed with the upgrade again. Earlier failed upgrade can leave your database in an inconsistent state. Hence, restore the working database before proceeding with the upgrade.
If you have made any customization to the default port number (which is 8888) in your current installation, you will have to manually copy it to the new psa directory and then start the upgrade. Else, use the URL as mentioned in point 1.
It does not matter if you click on startup.bat file from the install directory/psa_x.y.z/bin directory or from the command prompt, as both will have the same result. Its important that the window remains open during the upgrade process.

Even after following the correct steps mentioned above, your upgrade fails, drop a mail to support@celoxis.com with the debug bundle.

Steps to roll back to your earlier version if upgrade fails :

Delete and recreate the Celoxis database again.
Restore a working backup of the earlier database version.
Start the application from the earlier version.

Steps to restart the Upgrade:

Delete the existing (new) psa_x.y.z directory and unzip the directory again. Make sure you have unzipped it correctly. There should be a psa_x.y.z directory having directories like bin, conf, webapps, etc.
There should NOT be a psa_x.y.z directory inside a psa_x.y.z.
Delete and recreate the Celoxis database again.
Restore a working backup of the database. Ensure that the current version of Celoxis you are on is up and working correctly.
Start the Upgrade process again.

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