Hello @neva
After investigating your save game, the issue arises when you make more than one enquiry to the same club.
The game can only process one trade enquiry at a time per club as it is not a transfer offer for a specific player but an open negotiation with a club that could include any number of players, draft picks, finance etc.
If the player makes one enquiry to a club and then clicks to negotiate the offer, this will work correctly. If the option to offer anything in return is unavailable then the user will likely already have an open trade negotiation with that club that has not yet been withdrawn.
You can do this by going to: Transfers -> Transfer Activity -> Trades, and clicking on the red X underneath "Status".