Thanks, Empire. When I got mods down to 96 it worked again. I guess when you're close to capacity you have to account for mods on your current build being removed if you use the same gear pieces on a different build. What puzzles me is that I use the same mods on these gear pieces for both of the builds I was trying to use. One is an assault rifle build and the other is LMG, but the mods have perks for assault rifles and lmgs so I use the same mods for both. There must be a mod on one of the gear pieces that is different and is causing this, but if I'm not at full capacity, it shouldn't affect it. I hope they fix this.
Yeah. You need a buffer, especially in the mods inventory.
It's the way the coding allocates items. For example, if your gear item has all of its mods slots filled, and your unused mods inventory is at capacity minus one (99 items), and you PICK UP / LOOT a new mod (bringing you TO capacity), you can't simply equip it on said gear item. That's because in order for you to do so, technically you need to unequip the mod that you're swapping out, and that means placing into your mods inventory, which it can't because it's at capacity, despite the fact that our brains take the shortcut and think that there shouldn't be an issue, and that it's a simple swap.
The other thing is, given the drops, we all know that we hold onto a lot of junk in the hopes that some day it might be usable. We can all easily destroy at least one item from every category in our inventories and not miss them. If the item you just picked up is of greater value than some of the other items you've collected, don't hesitate to destroy one of them.
This is why increasing stash/backpack capacity really doesn't solve anything... because if you're inclined to hold onto junk, you'll be at capacity no matter what they increase it to. Incidentally, they needed to increase our stash capacity and loadout preset capacity to give us a buffer to adjust to all of the new items, sets, and specializations.