Bug report from the bug: In the battle arena your hp/mp bar is as follows: current/max
When you equip the ring of mana or ring of health, the current is affected, but the max stays the same. so you can have 20,000/10,000 hp. The bar is full, and say you've got 16,000/10,000 hp, you can't buy a healing potion(assuming your max is actually 20,000).
You need to fix the keypad. Not only can you not leave its view, but if you look closely, you can see which ones are the correct choices every time you click.