During the gameplay I've found a four chests that is not possible to open:

1. Maxos Temple, Eternal Maze: <click>
2. Barnabus lair: <click>
3. Kaera Fortress, frozen chest: <click>
4. Kaera Fortress, chest 2: <click> (<dragon view>)

If someone has any idea how to open these - do tell :P

Also, regarding to maxos temple chest: I have found a weird key but it seems this key not for that chest. Actually, I've found it after finishing the game, but I clearly remember that only this chest I wasn't able to open in Maxos Temple, so, I have no idea what opens this key >_>

P.S. Also, as addition, I guess there is worth to mention a two empty chests: one in Kali's Fortress location and one in Aleroth. Funny enough but they requires a high lockpick skill...