This doesn't just associate to weapons in armory chest. You need to check all parts of the armory and make sure you have suffecient room for your gear to come off completely otherwise that error message will pop up. So go through every part of the armory chest (head, legs, feet, hands, shields, neck, earring, ring, etc.) and make sure there is enough space to remove the gear as if you wanted to run around naked, then see if the error continues to persist. I can tell you right now from looking at that picture your problem is your body piece section of armory. Max slots for each Amrory section is 25, you have 25 slots out of 25 used in your body section. Try moving one from there then try to change class.