I am not optimistic that we will ever find out what really happened. The only meaningful investigation will probably be by Corendon Europe’s flight safety department, who I doubt will share their findings, (no blame/open) safety culture rules etc.
Whilst weird and wonderful failure modes of the pressurisation system and/or crew error cannot be totally excluded I think the more likely explanation is that passengers and crew were not at the top of their game with an early check in between two and three in the morning, if the flight takeoff time 04:27 on the internet is correct. It is reported by the operator that one passenger had an epileptic fit and was attended by a doctor. At the same time a number of passengers are reported to have fainted, triggered possibly by a fume event of some kind, notoriously difficult to investigate, but mass panic cannot be excluded at this stage either. With many years as a captain on the 737 I would be surprised if this was a pressurisation problem.
For me this scenario would be an ideal pilot interview question. You reach top of climb at five in the morning and the purser tells you that a passenger is having an epileptic fit. A little while later she says she has found a doctor but a number of passengers are distressed. The only airport officially open is your departure airfield half an hour behind you and the passengers seem to be recovering although some of your young foreign national cabin crew are having difficulty communicating with the passengers. What do you do next?