DSF

Where/how is your transfer paper stored? is there any chance it is a little damp? Maybe try pre-heating this to make sure it is also dry.

The paper shouldn't be damp... Humidity in the room is about 50%, paper is in the plastic bag. Before my next try I'll preheat it anyway, just to make sure...
But if the paper would be even little damp, shouldn't have I problems with other items? This sublimation paper I'm using for mugs, jigsaws, mouse pads and t-shirts - everything works fine except this acrylic blocks - pain in the neck :/ Already destroyed 3 of them, 10cm x 15cm

Just tried some YOLO papers for laser printer and t-shirt and had no trouble at all...