[2021] Certain DLC not enabling in Paradox Launcher

Are you playing from the Steam client or the standalone Paradox client (aka launcher v1)?  

Go to the Documents/paradox/stellaris/ folder, move any valued save games elsewhere, and user_empire_designs.txt from /Documents/Paradox Interactive/Stellaris/ if you have made any custom empires or races. Then delete that whole folder, does that help the error?  

Great, you can move those files back again now if you wish.

All the best!