In one of the first dungeons, that temple-ish sort of thing to the top right of the vale of shadows, there are some dudes that seemingly cannot be harmed with physical attacks - at least I keep getting the message '[character name] - weapon ineffective' and the dude does not want to die.
How exactly is this to be interpreted? Should I use a different kind of weapon damage? Maybe a missile weapon? Do I need to use magic?
And: why is my magician so shitty? 11 LP and only 2 spells to be memorized, that's ... not good ):

I think the DR system is only part of Icewind Dale II. The first game does have enemies who are resistant to certain kinds of attacks - bony undead against slashing or piercing, for instance - but I don't believe it will result in a "weapon ineffective" message.

So, yeah, a +1 weapon is what you need. At this point, you should have at least one. If you're lucky, you have someone with proficiency who can use it. If not, just give it to your fighter and have them go to town. If you don't have any +1 weapons, find Mirek's family heirloom on the Yeti Chieftan (in the outdoor area of the Vale) and return it to Mirek in town. His reward should be a +1 weapon of some kind.

Any weapon with +1 is considered magical! Now all my observations make perfect sense! That is the answer to my question!
My mage is level 4 has 15 LP and can memorize 3 level 1 spells.

Resistance was the original DR, and it can bring up the Ineffective Message. At level 4 the three best spells to memorize would probably be Sleep, Mage Armor and Magic Missile, if you got them. That is a Debuff, a Buff and an Attack spell.

I got chill touch that does (almost) nothing, fire something that especially burns my entire party to death and a bunch of useless abracadabra-nonsense XD well identify has proven useful every now and then.
Thanks for the hint, I'll go to the weird dude in the tower in Kuldahar (or however you write that) and try to purchase one of those spells you mentioned.
Have a good week (;

Burning Hands is a decent spell(its the only first level fire spell), Chill Touch is a Touch Range spell, so you gotta be up close. Most of the Abjuration spells are pretty decent at the first casting level(Not character level, Spell Level) but Magic Missile is the go-to low level attack spell.

Fire area spells work well together with Fire Resitance buff of clerics/druids. If you take 80% less damage, friendly fire gets acceptable.

Now that you mention clerics, what is the point of this 'turn undead' ability?

If you activate it (don't do any other actions with your cleric or else it switches off again!), you have a chance to "scare" all undead in sight, which makes them run away. On higher levels you can even instant kill lesser undead with it.

It's mainly useful for situations where you get swarmed by undead. If there are say 8 of them at once, and you scare 6 of them, you can first easily kill the two that stayed and then wait for the others to slowly come back to you.

That would have been useful in the bottom level of the temple in the vale of shadows...^^
Btw do you recall whether the skeleton mage there dropped a useful item? I had to flee after taking him down and when I returned, all objects were gone... ):

The skeletal mage there dropped the same as the other skeletal mages in the tomb, bracers AC8 and a random gem, nothing important.

Oh yeah I should have added that as long as you keep Turn Undead active, you will attempts to scare each undead in sight once per turn. So no need to click it again, unless you do another action with your Cleric.

It's basically a Cleric holding a holy cross in front of him. That's how I imagine it at least. By normal DnD rules, the undead needs to see the Cleric to be affected, but in Icewind Dale it always works.

Ah good so I missed nothing (important).

Is it like the bard's song, can the cleric not do any other actions while being undead?

A Cleric can not make other actions while Turning Undead. Otherwise they lose their concentration and the Turn ability stops.