error_log the url of every http request, to find out in which requests the issue
Well Apache should already do that, not sure what logging from the app would most help

Can you see a link between the requests and the error messages? I.e. is it always the cron requests like you suspect?

If instead of disabling our plugin completely, just disabling AutoCoupon
Well this is the functionality mainly used so not so keen on disabling it

Well, before you told me you were running with the plugin disabled for some hours so I hoped it was an option. Otherwise
you could try this in a testing environment. I think the only functionality of our plugin that could potentially cause an infinite loop would be the AutoCoupon in combination with other plugins that that use similar filters or actions, but this must be investigated in your environment.