When fighting Kahrk at the Firewine Bridge, as soon as the short dialogue ends, he casts Mirror Image but nothing happens, the spell animation instantly stops and he continues with casting Minor Globe of Invulneribility although he already appears with the Globe around him.

Yesterday when i met him the first time he appeared without the Globe and he did cast Mirror Image successfully.
But he killed me pretty fast so I explored the rest of the map.
But now when meeting him, the above happens and no amount of reloading changes it.

Anyone got an idea why? Is it a bug?

From what I recall his initial globe comes from the (undroppable) amulet that he wears. I'm not 100% sure on this, but this globe could have a % chance of protecting against spells (I think similar to the potion of magic protection that grants 50%). Therefore, his mirror image can sometimes bypass that globe. Personally, I have also seen several cases where Kahrk successfully casted Mirror Image, so it's probably not a bug.

Depending on at what stage you encounter him, it can be a very difficult fight, since he also has Protection from Normal Missiles casted on himself when he appears and he is hasted.

Apart from cheeses, you can pickpocket Bentan (at the other side of the brigde in the top-left corner) for a very powerful scroll that can be of great help against Kahrk.

Minor Globe of Invulnerability protects against all spells of spell level 3 and below.

Globe of Invulnerability protects against all spells of spell level 4 and below.

Though neither actually protects against Dispel Magic, but Dispel Magic cant dispel the globe itself.

Dispel Magic can dispel Mirror Image though, or any other combat protection spell (Stoneskin etc).

Btw if you have an Inquisitor in group, which IMHO is the by far best Paladin kit in the classic version of the game, they come with a Dispel Magic at double their current level PLUS they come with True Sight, which also dispels Mirror Image, and does so reliably.

In the EEs Cavalier is the best kit because the EEs fixed the spellcaster level for the regular Paladin spells, thus you arent stuck with mediocre spell levels and durations as in the classic game.