> for the past 5 days with several customers
As nothing has changed for month in the wppizza plugin and indeed the paypal one, the first question really would be what it is that changed your end about 5 days ago ?

that aside, have you enabled debug (https://docs.wp-pizza.com/troubleshooting/) and checked your debug.log ?

Are all of your other plugins/themes up to date ?

most (if not all) of these are symptoms not the cause.
if “ordervars” does not exist there is something severely wrong elsewhere
so , yet again, I need the (full) debug log
Your “suggested fix” would cause more harm than good

A few observations.
a) the debug log you sent me has none of these warnings you are referring to above in it
b) it does however have several hundred entries per day of PHP Warning: session_start(): Cannot start session when headers already sent in . PLease refer to https://docs.wp-pizza.com/troubleshooting/?section=headers-already-sent.
That said, I also believe this is more of the symptom than the cause
c) when trying to to verify the order at paypal, paypal respondes with a 403 error – i.e access forbidden (again, I think this is also only a symptom , not the cause)

What is definitely a problem – which may or may not be the root cause for everything but certainly one of it – is that you are caching you order page. Or more accurately, you are putting the checkout (widget) onto your article pages which are cached.
You must not cache any pages that have the checkout ‘widget’ on them or take steps to work around this (see also https://docs.wp-pizza.com/getting-started/?section=setup)
This is definitely something that needs fixing first. Once you have done that, see if you still have issues and report back

the debug.log you attached above is referring to a website where it’s all still as it was and you WILL have problems there

your ‘carrello’ , ‘/il-mio-account/carrello/’ urls may well be working but I cannot know as there’s no domain mentioned related to those in this thread.

https://pizzeriacarignano.it/il-mio-account/carrello/

there’s also a lot of stuff does does not work because of the way things are minified / combined
i cannot know what exactly is causing what, but I would start by disabling any kind of caching plugins you have running
(three also seems to be a siteground caching thing going on of some sort)

In short though,
– what does your debug.log say
– can you reproduce the problem consistently
– if so, do you have the same problem if you disable *all* non-wppizza plugins and use a default theme
(in other words see : https://docs.wp-pizza.com/troubleshooting/ )

Hi Olly,

Thanks a lot for your support and the detailed guidance — it helped a lot.

I wanted to confirm that everything is now working correctly on my side. I tested the PayPal integration in sandbox mode exactly as suggested:

I used the PayPal sandbox business and buyer accounts

Enabled Sandbox Mode in WPPizza → Gateways

Added the correct REST credentials (Client ID and Secret)

Performed multiple test orders successfully using the PayPal checkout button

In addition, I had to make some important adjustments to the cache and optimization settings on my site, which were causing the missing orders issue.

✅ Here’s what I did:
I excluded the following pages and URL patterns from caching:
/?wppizza-listener=paypalipn/
/?wppizza-listener=paypalreturn/
/il-mio-account/*
/checkout/*
/?paypal*
/ordina-online/*

I also disabled minification and combination of CSS and JavaScript for anything related to the WPPizza plugin, as I noticed serious issues triggered by SiteGround Optimizer’s combined scripts (as you mentioned).

At the moment, everything seems to be running fine again.

Thanks again for your help!

If the issue returns, I guess I will have to completely disable all caching and minification features on the site to ensure full stability.

Best regards,
Maurizio