I realized I hadn't saved my game after this error appeared, so I loaded it back up. Made a different but similar trade to the original one I made that started the issue (a first round pick for a CB), which was accepted and successful. The issue then came right back, and I can no longer make trades.
The first year of my franchise I rebuilt my team and ended up with a bunch of late draft picks. Over the years, I've been trading those forward and up, so I tend to make lots of trades involving picks. This almost feels like there's an internal counter for how many picks you can trade, and I've capped it out.
Anyway, this is quite irritating as my connected franchise is now essentially screwed - I have lots more picks than I need, and because the game reserves cap space based on your picks during re-signing, I won't be able to resign any of my players with expiring contracts. Then, to make matters worse, the game auto-signs picks, often with large bonuses, which will jack me for the foreseeable future.
Please take a look at this one, EA. There's a mode-breaking defect here.
Thanks.