So I was messing around with my team comp while the NA teams are still unlocked, and noticed that I could pull players from my team without needing to replace them. This leaves you with an empty spot on your team. I pulled my Team slot and benched it, and now it won't allow me to replace it. When I try to fill the empty Team slot, I get an error saying "The exchange would leave your team with an invalid starting roster, with too many players at one position."
Edit: Look in your Add/Drop tab, you probably have four alts now. Swap your team around until the person you want to put back is in the empty slot. For example, if you've got an empty ADC slot because you benched Mash without replacing him, swap your team around in Add/Drop until Mash is in your ADC slot instead of being an alt. Once he's back in his team slot, it should let you fill your empty slot on your roster.
Edit 2: This bug is actually even worse than I initially thought. Now I have 5 alts.