Dreaded Blank Page of Death

[johns4y]

Info: www.jaysmodelkits.com / ZC v1.5.4 / no plugings / PHP 7.3.12 / MY SQL 5.7.23
I set up my website 13 years ago and when looking back now I often wondered how the hell I got this far. Making changes to the website gives me the willies and updating Zen Cart sends my stress levels into overdrive. In a nutshell I am scared Sh1tless that I am going to muck something up.
I have been informed from a potential customer that the PayPal Check Out and 'go to checkout' result in a blank page. They were correct. I also found that, sometimes, when I click on 'back to shopping' this also results in a blank page, but not always.
I have searched and read as much as I can understand in trying to find the fault. My error logs went into overdrive on the 18th with this recurring warning:

Use of undefined constant MODULE_ORDER_TOTAL_GV_STATUS - assumed 'MODULE_ORDER_TOTAL_GV_STATUS' (this will throw an Error in a future version of PHP) in /home3/jaysmode/public_html/jaysmk/includes/modules/sideboxes/information.php on line 39

Since then I now get this warning come up:
Illegal string offset 'manufacturers_id' in /home3/jaysmode/public_html/jaysmk/includes/classes/navigation_history.php on line 40

I get many variations of this one where the 'manufacturers_id' can be different each time.

I need help and fast. I believe the blank pages have been around since at least the 14th Dec, the date of the last successful order we received, or it first started on the 18th. I just don't know.

If anyone can help me get my shop up and running again that would be the best Christmas present I could with for.

[swguy]

If you are running 1.5.4 you will need to crank your PHP version back to 5.6. Then get working on the upgrade to 1.5.6 so that you *can* run PHP 7.3.

[mc12345678]

Recommended instruction for such an upgrade would be: http://www.zen-cart.com/entry.php?3-...d-of-upgrading