Moemoe <3 I've gotten this a few times. Send a 1:1.
This error is caused by your items disappearing for some reason, and you're able to pick up more items. When the missing items come back, you have over 300 items. >:
If you didn't have any cards on your character, be sure to tell them not to move any fused items. They moved 3 hats of mine before, and they went back to the hood image, and I had to 1:1 again to get it fixed.
You should be able to log into your other characters on the account, though. I was able to.