Since I made the update to version 2.8, the portrait of the player character appears as if it had a shadowing filter applied over itself. Does anybody has this same problem or have found how to solve it? In advance, thanks.

I have this same problem. When I mentioned it at Steam, some developer representative posted and just said "I've never heard of this. Put a ticket on Redmine." I hate when a company's answer to every problem is "Just put it on Redmine." Nothing ever comes from that. I'm skeptical this problem will ever be fixed, because it apparently isn't affecting enough people's systems for it to make a big enough stink.
I decided to just tolerate it, because the graphics are so much sharper with the DX 11 version that causes the portrait darkening. If you really want your portrait back, you can play on Steam in DX 9 mode (not sure if there's a DX 9 mode on GoG), and that solved the problem for me, although I thought the graphics were noticeably worse, so I went back to DX 11.