Further update:
Since disabling the caches, the incedence rate is much lower. However, we have still had it happen a few times. This suggests that whilst caches may exacerbate the problem, it is not the root cause.
With today's occurrence, there was no error generated on the server. The log just showed the handover to the payment module but nothing else:
- 81.108.18.251 - - [31/Oct/2020:13:15:44 +0000] "GET /order?step=1 HTTP/1.1" 302 0 "https://mms.payzoneonlinepayments.com/" "Mozilla/5.0 (Linux; Android 9; ONEPLUS A6003) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.114 Mobile Safari/537.36"
- 81.108.18.251 - - [31/Oct/2020:13:15:45 +0000] "GET /cart HTTP/1.1" 200 15890 "https://mms.payzoneonlinepayments.com/" "Mozilla/5.0 (Linux; Android 9; ONEPLUS A6003) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.114 Mobile Safari/537.36"
Apparently the cart/payment was open a bit of time - could this be a factor? (And why the problem is far more prevalent when caches are enabled?) Either way, we know it is an issue with either the PS Code, or the module. But if it is the module, it happens with all our payment modules (suggesting they have been developed to a certain spec that now is slighjtly incompatible with 1.7.6)
Any idea on this? It is driving me crazy