once i reached lv 12 mage (lv9 Kensai)
one spellslot of each level disappeared.
Is this a bug or something
(i've got the patch installed, Is this the problem???)
Sounds like the game thought you were a specialist mage for a time, thus getting one additional spellslot on each level, before suddenly realising that you can't dual class into a kit in BG2. Pretty wierd bug. Do you have the normal amount of spells for a mage now or are you one spell short on each level?
Besides using an editor to correct that problem I have no idea. Try leveling up first though. Sometimes the game has a way of correcting itself with sich things.