I wanted to provide my own data point. About a week ago I had the same issue of TK canceling my seat selections. I sent TK feedback and emailed my local sales office, and still no resolution. My flight appears to use the Aeroflot A350 aircraft.

Now 2 days ago the TK website doesn’t even allow me to see the seat selection screen. When I search on Expertflyer, an error appears. Lastly, I noticed this error every day the Aeroflot aircraft is flying.

Maybe TK is experiencing a technical issue with this specific aircraft seating? They may also be in the process of swapping aircraft’s (which I’m dreading). Not sure what’s going on with TK…