first of all, without a url where I can see things, I cannot really comment I’m afraid.
Furthermore, your third screenshot above is also missing the settings at the top of that page, which have an influence on how things work.
Lastly (for now): is the plugin up to date / latest version ?

> On other pages, if this above function is deactivated, the page works fine. And generally, I use the same theme and plugins everywhere.

I’m confused now . You have this activated from what I can see and it works on your site just fine now ? (And I did not change anything )
So what settings do I need to enable/dissable to get yoru problem please to reproduce this.

PS: you still really should address the session error as mentioned in my post above..

> However, on the order form, zip codes can still be selected.

of course. that’s the idea ….(?)

not withstanding my previous reply (i.e try that setting and I will see what happens here too) the seesion issue needs fixing too your end
see
https://docs.wp-pizza.com/troubleshooting/?section=headers-already-sent

>I will also send you the log file later.
ok
have you also deactivated all other plugins / used a default theme ? (to narrow things down and be shure)

Could you perhapos also post a screenshot of your postcode plugin settings ? thanks

first thing I would suggest is turn off your caching plugin and / or also re-save your plugin settings
furthermore , enable debug (https://docs.wp-pizza.com/troubleshooting/) and check your debug.log
let me know the outcome once you have done that

then your hoster cannot read , cannot be asked or doesn’t know what he is talking about . (or possibly all three)
It clearly says in your debug

[25-Aug-2023 00:54:18 UTC] PHP Notice:  session_start() [<a href='https://secure.php.net/manual/en/function.session-start.php'>function.session-start.php</a>]: ps_files_cleanup_dir: opendir(/var/cache/php-sessions) failed: Permission denied

i.e the session cannot be started because the permission is denied to where the sessions are stored. this has 1000% zero to do with the plugin and all to do with the server setup. Updating the plugin will do absolutely nothing

As of 3.18.1 the plugin also includes a localised version of the “smoothness” css (named “smoothness.gdpr” as an option in the theme dropdown choices ) you could select instead.
However, there will not be any other localised versions of any of the other themeroller themes so if “smoothness” is not what you are looking for but still want to load a localised version , you will have to provide your own css as outlined above

just mentioning it here for completeness sake , in case someone else has the same requirements.

The downside of this is of course that this local version might not always be the most recent style you would otherwise get when using the external css file hosted by jquery.com….

A decision the user will have to make in the end . I’m just offering the option now….

you would need to set “WPPizza-> Order settings->Items update style” as well as
“WPPizza-> layout -> miscellaneous -> accordion style” to

“a (themeroller) style is already loaded / I provide my own style”
and provide your own style/css somewhere (or not)

if you entered the licence key and activated it (WPPizza->Tools/Licences : Licences-Tab) you can update it from the plugins page of your wordpress installation.
Alternatively , you can download it from your accounts/purchases page

https://www.wp-pizza.com/my-account/
https://www.wp-pizza.com/checkout/purchase-history/

PS: The preorder widget will not work if you do not use a dedicated orderpage (see also setup https://docs.wp-pizza.com/getting-started/?section=setup)
You seem to have put the whole orderpage in the sidebar. Although in some circumstances this will work , it limits a number of things you can do so I would suggest you create a dedicated orderpage (although I’ll have a look here if I can make the preorder page work there too and will update it I can forn a workable solution)