Hi Herve,
Yes, I agree - and BTW it's far too late to be answering support queries :) However we had some pretty bad experiences in the last few weeks with huge volumes (first free event we offered ... ha ha !).
Unfortunately there are many moving parts as we have maybe 50 components running on the site, and it took testing since late January to narrow down the cause of the Free orders problems to the Stripe plugin, but even after further testing then revealed some other symptoms when the volume came on recently.
Anyway, I've just completed smoke testing a combination of Joomla 3.8.4/VM3.0.16/Stripe 2.2.1 running which looks stable, albeit with the problems of the Free payments method. I re-installed and re-enabled the Stripe 2.2.1 plugin and everything else looks OK. Visa/Mastercard/Amex/Paypal are working successfully, backend and VM tables look fine. Free payments still has the issues we discussed earlier.
So I'm going to leave it for tonight and clean up tomorrow, hopefully the Free payments issue is an easy one :)
All the best,
David