I've passed the info on including network traces.

If you are sending multiple contract offers by clicking on a player, then from the player card selecting the offer contract option, the years and requested bonus will be populated with the previous offer you made to any player (not tested if you logout then back in).

WORKAROUND: Drag slider bonus to 0%.

This forces a call to the server and you get a message like this :-

A multi-year offer requires a bonus
This player will not renegotiate for a bonus less than $13,782,447
A 4-year contract must include a signing bonus of at least $197,576

$13,782,447 is the true minimum bonus. Manually enter that into the bonus box.

Testing has shown that this problem ONLY occurs when offering contract through the player card. If you click the player to get the card up then select View to get the full player page displayed, this problem was not observed when selecting offer contract from there.