Hi, I have checked your account and both defensive and offensive rings can be put in the empty slot. Can you please specify what exactly you do and whether you receive any errors? 

Maybe the issue is caused by the browser, please try to clear the cache and check if it helps 

ispec34 said:

Since a few days I cannot put a ring in the offensive ring slot of the hero item's anymore. It's working fine in the defensive slot.

Either ring can be equipped on either slot, it makes no difference. The game will normally equip your rings on the wrong slots, just ignore this bug.

You can also equip 2 identical rings to temporarily boost offense or defense to a maximum of 70%

If you are having problems then just Drag and Drop the equipment

The interface is bugged, but it does not need to be fixed because they will just create new bugs