1.6.1.24 to 1.7.8.3 1-click upgrade won't work, manual upgrade not working

[JCPS]

I tried using the 1-click upgrade; however it indicated I already had the latest version for both prestashop and the 1-click upgrade module itself.

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The 1-click upgrade module is up-to-date (your current version is v4.14.0)

Congratulations, you are already using the latest version available!

Your current PrestaShop version: 1.6.1.24

Your current PHP version: 7.3.33

Latest official version for minor channel.: 1.6.1 stable - (1.6.1.24)
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I then tried doing a manual upgrade as per the instructions here: https://devdocs.prestashop.com/1.7/basics/keeping-up-to-date/upgrade/

After uploading and merging the new files, I was getting a 500 error on everything. I turns out all the files and folders in the 1.7.8.3 ZIP file have the wrong permissions 664 and 775. I changed the permissions to 644 and 755 and that fixed the 500 error.

When I ran the script to upgrade the database <shop_domain>/install/upgrade/upgrade.php, the page seemed to be working, but then just loaded blank. Trying to access any page after that would just load a blank page.

I ran the install/upgrade/upgrade.php a second time and this time it showed this:

===============
XML Parsing Error: no root element found
Location: <domain>/prestashop/install/upgrade/upgrade.php
Line Number 2283, Column 1:
===============

After this, when I try to access the back office, it shows the log in page, but after I enter the email and password and click login, the login box just disappears and it remains on the same page.

What am I doing wrong? What else can I try to fix this?

Thanks.

[JCPS]

For anyone having the same issue with the blank back office login, this was solved by updating to PHP 7.4 (7.4.28 to be specific).

I previously had PHP 7.3.33, which is supposed to be be compatible according to https://devdocs.prestashop.com/1.7/basics/installation/system-requirements/ but for some reason that was not the case.