Kkkkkk Posts: 1 Just Dropped In
According to announced event rules:

Starting with the release of Amonkhet, all the events in the game will now have new rules in effect that limit the card sets available to participate in the event.

PvP Events from Kaladesh, Aether Revolt and Amonkhet can now only use cards from Origins, Kaladesh, Aether Revolt and Amonkhet combined. This means that Fate is Rarely Fair, for example, can now only be played with cards from Origins as well as the Kaladesh and Amonkhet blocks.

Note that events from prior sets as well as current PvE events have no restrictions on card sets, and you can build decks using any card in your collection.

PvE Events are those against pre-designed opponents, if I get it right.

However, Note for PVE Events does not apply to Trial of Zeal, but new rules for PVP Event do!
I'm not sure, if it should be reported as issue. 

How do I gain enough Mana crystals to buy new boosters and to be able to play as I used to, if it is practically impossible to win?

This does not mean equality, this is just unfair.
When I started playing I did not have as many cards as others, but I was playing in PVPs against Planesworkes with the same level as mine. That was fair, but looks like that is not enough anymore.