I found a plain Mastery scroll consumable dc2_masteriesscroll, and then lost it later trying to solve a crash involving it. From its description it sounds like it is the one required to enable mod's function before attempting to learn other skills. Using it on a Lvl 25 class without any Mastery skill learned triggers a game crash.

For a detail, my other mods are "Unlock All Spells" and "Tiered Passives".