Well firstly the web shield doesn't scan HTTPS pages as they are encrypted, the problem arises when you move between HTTP to HTTPS and back to HTTP as there will be parameters passed between these pages.
There is a similar issue like this with Zone Alarm Pro with its privacy function as it stops this transfer of parameters through the web shield proxy (localhost port 12080) and when you try to load the end page it is expecting these parameters and without them the page fails to load.
Now I have no idea how your Linux server filters traffic traffic in a similar way to the above, but I don't know if simply excluding the page would work as a) we don't know the page that is causing the problem and b) the traffic would still be filtered through the web shield proxy, just not scanned. So I don't know if it would work see below.
I don't know if you can set your Linux server/firewall to allow ashWebSv.exe and any localhost traffic on port 12080.
Exclusion/Ignored:
There are two options the first I think will be the most promising.
1. Web Shield, Customize, Basic tab, Ignored Addresses: enter the IP address for the domain. Whilst this is likely to work it does leave a hole in the defences as it ignores all pages on the site.
2. Web Shield, Customize, Exceptions tab, Add... button enter the URL address for the page you wish exclude/not to scan. I personally don't think this is going to work as there isn't a scanning problem but one of the parameters not being passed (and that isn't down to avast as it isn't a blocker).