For quite some time i was missing Card Backs from Solo Adventure Challenges which i completed and had in my collection before. Support was not willing or able to help me with that matter.
Notably i was missing the following Cardbacks:
-Boomsday! Card Back from Boomsday Project Puzzles
-Monster Hunter Card Back from the Monster Hunter Solo Adventure
-Rusted Legion Card Back from the Trial by Felfire Story Mode
Since there didnt seem to be a solution i tried to just earn them again by completing the respective Solo Adventures but they are bugged for me.
-witchwood: i cant play the final challenge. it says i have to play monster hunt first, but i already have completed monster hunt and it doesnt let me progress there again.
-trial by felfire: i cant progress after the prologue it just dont lets me play normal missions. also i have later missions in the story mode marked as completed which should be impossible
TL;DR: Missing card backs which i had before which i cant reearn since i had prior progress in the respective story adventures