I'm using XCR coating resin since last year. And never had difficulties to sand it dry. If it was hardened for at least two days, dry sanding always went smooth. But I've experienced the opposite this Summer. When it was impossible to dry sand resin that was hardened for 4-5 days. It clogs sandpaper or Abranet immediately. Also noticed weaker adhesion properties of that resin, no matter that previous coat was sanded with 220 paper. I have suspicion that it has a lot to do with air humidity. But maybe there are some additives available that would help to avoid such problems ?

In my opinion, before you can start getting to this conclusion, you have at least to test resin from the same mix side by side on two different surfaces, keeping everything else the same. There are too many other things that can interfere in the process.
If it was bonding to the surface, I would totally see your point there.
In my opinion epoxys are really reliable. If I done everything right it never failed me. Bonding, pot life, curing, strength, every aspect. So far every batch behaved just as the one before. and the specs were always right. the only thing you have to get right is the mixing ratio, the mixing itself, temperature and the underground preparation in terms of bonding/releasing.
Polyester however is much less reliable, wich is why I do most of my moulding in epoxy, too.

When somebody comes to me with incurred epoxy, in the most cases, they mixed everything by eye, or their interpretation of 100:30 is "out of every 100 units, 30 should be hardener", wich is what we would call 70:30.