Cause is conflicting asset, namely Content \ __ExternalActors__\ThirdPerson\Maps\ThirdPersonMap\D\SY\Z4CTQ4LG3YV10EKPD0UE8Q.uasset
It happens when you try to install asset pack which has demo made of Third person template into project that itself is made from that template or already has some other pack with similar-made demo. Launcher stumbles over this conflict and probably wants to consult Epic about what to do but Epic keeps silence (as usual 
) so launcher awaits.
You may find and delete this file in your project (backup for safety first) and retry to install the pack, usually it goes well.
Alternatively you may install it manually by copying files from AssetVault and overwriting this and possibly other conflicting .uassets
Hope @SkyeEden will pass this info to proper dev.