Missing order numbers

[mathm00]

On two occasions we have order numbers missing. First instance on Januari 23rd. Two numbers are missing from the order number sequence. A customer reported difficulty with getting a order completed. When i tried myself to place an order all went well.
We use:
Joomla 3.6.5 and VM 3.0.18 and have all (security) updats installed. We use a payment provider.
We are looking for clues what could be the problem.
Simultaniously we have an 'email issues'. We don't know it this is related. Just mentioning it because maybe the two problems are connected. Sometimes when a status is set to shipped and customer is informed, as administrator i  receive the conrfimation email of the order ( listing the details of the order) all over again 1 or 2 days later and at the same time the status of the order at backend is reset to confirmation of the order. The shipment confirmation information that was send to the customer, is then no where to be found anymore. Gone with the wind.
Thank you in advance for responding.
Annette

[Studio 42]

Backup your site and try VM 3.0.18.6 can be downloaded here : http://dev.virtuemart.net/projects/virtuemart/files

[mathm00]

Thank you for answering. We wonder why this specific version 3.0.18.6 and not earlier or later version? I understand it is labelled as release candidate and save to use live in production?
Had you heard of these problems before?

[Studio 42]

I only know that 3.0.18.6 is more stable.
I never had such problem myself.

[Milbo]

Either vm3.0.18.6, because it uses the old category layouts, or use vm3.0.19.7

[GJC Web Design]

Sometimes when a status is set to shipped and customer is informed, as administrator i  receive the conrfimation email of the order ( listing the details of the order) all over again 1 or 2 days later and at the same time the status of the order at backend is reset to confirmation of the order.

I have seen this on a couple of sites and traced to the payment plugin (I think in one case it was Heidelpay ) not checking the current status of the order when a repeated IPN was sent.
There should be a reality check to see if setting to Confirmed makes sense or just ignore
The IPN's sending repeatedly was from Google Adwords in one case and in another I think from a stored url in a browser session that they were re-opening.. (was a while ago so details a bit hazy)