I recently started a new game, including all official DLC, the unofficial skyrim patch, deadly dragons and a couple more mods.
Everything went well until I beat Mirmulnir, I didn't get to absorb his soul, he just lies there, busy being dead. All the guards around were hiding/crouching as if I was absorbing the soul, but nothing happened. A reload from an earlier save didn't help. This resulted in the mainquest being effectively stuck.
If I play in another game and kill a dragon there, I still can't absorb his soul.
Best Answer
Do you also have the Unofficial Dragonborn Patch installed (separate from the Unofficial Skyrim Patch)?
If not, install it. One of its fixes is:
According to the Unofficial Dragonborn Patch's mod description, the load order should be like this:
Also as a reply to this answer, you shouldn't load Dragonborn after USKP, as it could undo the fixes of USKP. If unsure, just let LOOT take care of the load order.
Install the other Unofficial Patches if you have their DLCs too: Dawnguard, Hearthfire, High Resolution DLC.
Also, see the troubleshooting steps for the 'cannot absorb dragon souls' issue listed at the Unofficial Dragonborn Patch Nexus forums sticky post by Arthmoor, one of the UDGP developers: