I cannot respond to offers for player transfers in career mode.

I've had this happen as well and it's frustrating.

Van Dijk - Complained about being left out of the squad after 5 games in a row to help rotate the squad and recover fitness.  He then submits a transfer request.  I could negotiate transfers until the January transfer window, then I couldn't any more.

I had 2 other players with less than 12 months on their current contracts that I had on the transfer list.  I could accept and negotiate transfers until both requested to sit down and negotiate a new deal (after going to the board and the board telling me to sort it out).  After the failed negotiations I couldn't accept transfers.

While this is frustrating, I've got two work arounds:

1. Not all player messages give you a notification - manually check so players don't get stroppy and submit a transfer request.
2. If a player has submitted a transfer request and you can't negotiate, try and use them as swaps in transfers you initiate.