"0 - Cannot call constructor" error at checkout

As of yesterday, customers are getting this error at checkout on my shop - feralangels.com

The server logs show this kind of error:

2024-12-08 11:05:35.351763 [INFO] [390288] [T0] [114.119.149.169:62089#APVH_feralangels.com:443] File not found [/home/feralangels/public_html/403.shtml]

I ran one or two plugin updates yesterday morning that coincided with this error occurring - it doesn't seem to happen anywhere but on going to checkout.

Anyone have any ideas?
Giang - I've opened a support ticket, but am hoping for a quick response as this is the last day for international orders via the shop

(to make matters worse, my backup from before the updates failed )