The bug only occurs if you try to offer mods that you maxed out inside the dojo. If you level them up before joining, you can make the offer just fine. That's what I've worked out so far from my testing, because I ran into the same issue today. This is also the reason you can offer equipped mods just fine, because they were already leveled up before you joined the dojo. Anyways, this new bug is really annoying.