We were unable to verify ownership of this DLC - How to fix it?

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Please do a full **clean** re-install:

- 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.
- "uninstall" in Steam-Stellaris
- manually delete both the Steam/SteamApps/common/Stellaris AND Documents/Paradox Interactive/Stellaris folders
- re-install game in Steam, run a Steam Verify when done.
- add stellaris.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test

In particular you need to make sure there are no files or folders left under either the Steam Stellaris or Documents Stellaris locations, before installing again.