Problem with some tickets not registering, even after paying

**Describe the bug**
I'm part of a student organisation which organises several events throughout the year with ~150 attendees. Using this platform has rarely resulted in issues for the students. Recently we launched the sale of a bigger event with other student organisations (650 tickets). These tickets tend to sell out within 5 minutes so it didn't surprise us that the site went down twice during the sale. After this concluded we got several mails from students that they never received their QR-codes even though they paid and arrived on the /succes page.
Checking their id's gives no results in the reservation page nor do they appear to be in the cancelled list. Are there concrete steps we can undertake to resolve this and/or prevent this from happening in the future? I noticed that 19 tickets are stuck on "pending" on the homepage of the event (which is give or take the amount of complaints we got), even though I can't find these in the reservations tab. Hopefully that gives a clue on what happened here. I'll put the screenshots down below. Thanks in advance.

Version: Alf.io v.2.0-M5 using docker-compose

**Screenshots**