WooCommerce not sending transactional emails

[ Moderator note: moved to How-to and Troubleshooting. ]

We are struggling with this problem for months and we hope to find someone who can help us with this.

What is working?
We use Ginger payments for iDeal and Banktransfer and both customers and admins receive emails when an order is made through Ginger.

What isn’t working?
The problem exist with Quickpay.net (payment plugin) and the integrated Paypal option. Both admin nor customers get any email.

The connection between all payment providers seems to work because orders from all payment providers are updated when payment is succesfull.

What we have already tried:
– Make sure there are no points or comma’s in the name of the sender (WooCommerce email settings)
– We installed a mail log plugin. Conclusion: no errors here, mails don’t even get sent so there is no reference in this log.
– We have tested a SMTP plugin and mail is then being sent over SMTP but our problem remains.
– Deactivating all plugins.
– Switching to twenty sixteen theme.
– Contacting Quickpay. Response: We are not the one sending emails so we can not help you.

I don’t know if this has something to do with our problem, but to make things just a little more confusing:
– Simply browsing to the callbackurl of the WORKING payment provider gives us a 500 error.
– Simply browsing to the callbackurl of the NOT WORKING payment provider gives us a page displaying “-1”.

We believe this has something to do with the callbackurl from Quickpay.net (payment plugin) and the integrated Paypal option.
I do not have enough experience with api’s to fully understand the workings of them, but is there a way for me to test what doen and does not happen when a callback is made?

And can anyone tell me when, in the whole process of ordering-paying-callback, the email to admins and customers is sent?

We hope someone can help us out with this. Many thanks!