Originally Posted by TrollanKojima
For anyone who needs it:

This issue - in a lot of cases - seems to be related to leftover Patch .pak files from before Patch 4.

- Navigate to your Baldur's Gate 3 Install Folder's "Data" folder.
- Find every .pak starting with "Patch" or "Patch_"
- Delete them.
- Restart Baldur's Gate 3.

With Patch 4, most of the pre-existing patches are now referenced in the PsoCache file. Deleting the Patch .pak's themselves seems to solve the invisible character/missing main menu button issue.
This has been a confirmed fix for many Nvidia users, and a multitude of AMD users.
Dear elders of the internet.
Once again you saved my mortal soul with your endless wisdom!
This solution did the trick for me. Everything is back to normal!
Thanks again!