As the developers have decided to push out countless new characters and souls we are now at an interesting point in the game.

You can reach a point where you are both over the soul limit and the character limit. The soul error requires you to open souls before doing anything else but if you open enough souls to push you over your character limit without opening enough to get below the soul limit you are stuck in a loop of "Open souls before doing anything" and "Sacrifice/Sell Characters before opening souls".

I was eventually able to click fast enough to get into the store between messages and then quickly purchase sac hearts for 300 common souls in between the messages again which still kept me over the character limit. But what would I have done if this didn't happen during a sacrifice event?

This is just one of the many reasons why increases in all inventory slots are no longer QoL updates but necessities for the long term players.