I seem to have problems dualing an archer to cleric. How come? Is there something I have missed? Being a ranger kit it should be allowed as far as I can see. Energy Blades and Called Shot could be wicked.

Not allowed, possibly because dualling to a Cleric would restrict the Archer from the types of weapons Bioware obviously meant for him to use (ie, the ones he can attain Grand Mastery in).

With Tactics it is possible to make an Archer/Cleric by using the Generic Archer. You lose out on the druid spells, but on the other hand you can attain Grand Mastery in slings, which offers the possibility of slinging 50 dmg bullets at a speed of 8 ApR.

Sometimes the logic Bioware applies strikes me as odd. The fundamental idea of dualing is that you restrict yourself in one field to expand in another. Like you say slingers are perfectly viable archers.
Alas I think the archer was build on the idea of the elven bowman and as such I guess I must agree. A restriction from bows would ruin that.

The way BioWare arranged it, only Beast Masters and unkitted Rangers are able to Dual to Cleric, Archers and Stalkers are pretty much stuck.

But I'm of the opinion that BioWare may have made a few mistakes (gasp!) while programming BG2. I say that Archers should be allowed to Grandmaster in Slings and Darts as well as Bows, and Dual to Cleric just like regular Rangers. And what's wrong with a Stalker->Cleric that beats people's heads in with a Club or Staff? Sheesh.

Well I agree there. Slingers are the historical archer of many armies and are part of many fantasy universes as well (the slingers of the Thieves Guild in Lankhmar for instance). A weapon like the staff sling is completely lacking from the game.
Many will specialize their Stalker in staffs anyhow since THE backstab weapon is a staff (SotR+6).

Backstabbing with a staff... what will come next?

Backstabbing is an incorrect term. It should be "sneak attack" which always applied in D&D, but the term was never changed until 3rd edition (BG2 is 2nd edition).

Yeah, sneak attack sounds better when you use a staff to do it anyway...

if somebody sneaked up on me and whacked me over the back of the head with a staff, i wouldn't be happy

Actually it was called backstab in (2ed) D&D as well. From the Rules Encyclopedia page 23
"Backstabbing: If a thief can sneak up on a victim, completely unnoticed, the thief may backstab - if he is using a one-handed melee weapon, he may strike at particularly vulnerable points of his target's body. (Though the ability is called backstabbing, the weapon doesn't have to be a stabbing weapon. A thief can use this ability with a club, for example.)"
Rules Encyclopedia is a condensed format of the old boxed sets. Remarkably the 2ed AD&D uses 4 times more space to say less. In fact there is a special club just for "backstabbing" called a blackjack with its own seperate mastery table. In many ways 2ed D&D is more advanced than 2ed AD&D.