Error on products that have a discount

Hello.
I use Opencart Version 3.0.3.6
The theme is Journal 3. I have nothing to do with coding, but I don't think it's related to the theme.
Basically, when I click on any product that has a Discount set, from the Discount tab, I get the following error: https://prnt.sc/vz3fpm
There is nothing else I'm doing apart from setting a Discount.
I just don't know what to do. I tried restoring older backups in Opencart aswell as in my cpanel. I ended up in writing a ticket to my host and they restored a previous version of all of my files, including the databases, but with no success.
I did search the forums for similar problems but I couldn't find any apart from This thread . I have consulted with a friend who knows a bit of coding but we couldn't manage to fix the issue.

Please help
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It looks like you have one or more extensions that modifies catalog/controller/product/product.php. Disable your extensions one by one (and refresh) to track down which one could be causing the problem. If it's not your extensions then it could be your theme. The Journal makes lots of changes to OpenCart, which could introduce bugs or incompatibles with other extensions.

I have found what caused the issue.
The extension is called "[underr] Total Order Discount" and apparently that is what caused the issue.
Thanks x lots for guiding me @ADD Creative, you the man!