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 ?

You simply cant draw conclusions based on two results. Test it 20 times with varying batch sizes and I would probably support your conclusion, but otherwise I have worked with dozens and dozens of different types of resin including in the customer support sector. I have had many many people bring products back and say it doesnt work, insisting that they didnt make a mistake and that there is something wrong with the resin - not once has that ever been the case. It is always one of three things - off ratio, poor mixing or low temperature. Other than that, the resin that comes back to us always works when we do a test ourselves. In 19 years, the company I work for has never ever had a single batch of epoxy resin that didnt go off due to a fault of the resin. Polyesters, vinylesters, polyurethanes, silicones - sure, all of these go wrong from time to time. But not epoxy.

I'll rephrase my previous statement; its possible it isnt coincidence. But I dont think you have anywhere near enough data to prove anything just yet, and I think it would be a shame for you to drop a good product from an excellent company based on two results.