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 ?

If we are in technical science discussion we should avoid that word. Because what you call coincidences, in science world are facts that were not taken into account or simply missed.
I have experienced twice last year that XCR resin does not cure on MGS resin. If to be precise, it was MGS L285+H286 ( it's stable product itself, I used it since 2012). After first time I called it coincidence. After second it became proven fact . It's not very funny to scrub resin off and then try to wash it using acetone. So it stays in memory for long. 
What about recent issues. I had two SUP boards. Both were laminated using EL2 resin and then coated using XCR resin. In both cases XCR did not cure properly resulting in sanding problems. After first I thought it maybe coincidence . After second I've written to this forum.
I had one surfboard project at the same time. Laminating was made using XCR and then coating using XCR. Cure perfect, no sanding problems . I do sanding using sanding machine with MIRKANET. Usually it is hard to clog because there is dust suction through whole surface. If it clogs that means sanding material is too soft for dry sanding.
oekmont, all scales I'm using are 0,1g precision. You must agree that when mixing batches of 150-250g the precision is around 0,2%