When I level up I get skill point.
I can raise my skill from 0 to 1, but not higher (there is no + behind this skill when I level up).
But one of my followers has level 3 and is a watermage skill 2.
My char does not need more skill, but he wants to increase the skills he has (like more 1h damage).
I am also level 3, but I cannot raise any skill except from 0 to 1.

Sorry I forgot the correct in game terms for this. I hope you know what I mean.

To learn level 2 of an ability you need 2 ability points, level 3 requires 3 points, etc.

In my experience (my characters are level 20) there is no 'etc'. You are never granted more than 3 points to spend, and therefore cannot level past 3 base points in anything. The rest must come from traits/equipments/accessories etc. I get irritated with this, because I read people online saying that their characters have 21 in such-and-such a skill, and I have never been able to get anywhere near that. For example in telekinesis my highest is 6 (3 base, 2 rings, 1 gauntlets -- those are the only items that come with telekinesis boosts). If higher values are possible, how?

You don't have to spend those points right away, so at this point I'm assuming you can save up more than the amount you get from just one level-up. Hopefully I can level tonight to verify that points can be saved for future use.

Groan! Please don't say I have squandered 19 level ups. I will not be pleased. I'm already losing interest in this game the further in I go. At this point I have completed all the quests (63) I can find; found 16 star/blood stones (still have all the inert ones), and I am STILL refused entry into the source temple, telling me to find more stones. I can't find anything else, let alone stones! I spent three hours today -- three fruitless hours -- scouring the the maps for *something*. Nada. My homestead is still under attack, so I only have 5(?) rooms unlocked. I'm at an impasse.

Do you have any that are not inert? If so, use them.

I think if your homestead is under attack, you need to stop the attack first, and open the portals.
After the attack is dealt with, go to your homestead and open a portal, leave homestead, go there again, open another portal etc. (afaik you can open 1 portal / visit)
The last portal "Forbidden Zone" never opened for me. Party animals go in and out, so I guess they have a party there, and I'm not invited :P
The last portal I opened led to a chapel with a potion shop. After that the door let me into the temple.
(I didn't even need to use up all my bloodstones......)
(and yes, you can (and sometimes have to) save the points after levelling up =/ )

That didn't help, even after I closed the rift. I managed to close the rift and unlock lots more rooms. No need to leave, just follow Zixzaxx around, and they all unlock. I don't have that portal (I have an interdimensional trader), still can't get into the temple, and I still can't find any more quests to continue. AAAARRRGHH!

If you're under attack, as in the fireballs thing, then you need to take care of that. North in the desert there's a demon area, you need to go through that and take care of the void stone at the end (with a Tenebrium weapon)

Hmmm... I can only think of two easily missable star stones here:
- you can dig up one in the Source Temple, left from the broken mirror you couldn't use
- talk with the Iron Maiden in the prison you found the White Witch at, and destroy it to get a stone

I'll look up where the other stones are.

EDIT:
Found a list of star-blood stones in the design documents

Cyseal
-Star Stone on ground near exit to Luculla Forest
-Star Stone in the crime scene in the tavern
-Star Stone dropped by Braccus on death
-Blood Stone owned by Evelyn
Black Cove
-Blood Stone in the Pontius Pirate fight
Luculla
-Star Stone in the Immaculate Trial
-Star Stone on Mangoth if he hasn’t sacrificed (becomes bloodstone if he has sacrificed)
-Blood Stone in the Maradino Lair
-Two Blood Stones in the Loic Cellar
-Blood Stone when destroying the totem in the goblin village.
Hiberheim
-Star Stone in the Boreas treasure room
-Blood Stone when destroying the Iron Maiden in the prison
Dark Forest
-Star Stone in the tomb at the end of the knight’s tomb
-Star Stone buried in the source temple near the teleportation mirror
-Blood Stone near Cassandra’s actual corpse in the Temple of the Dead

What do you mean that you couldn't target the stone in the knight's tomb? On a side note, the quest, "The watch is coming" can be solved in the Knight's Tomb. You meet two people from The Watch there, they take the imps away.

Slave-master quest - did you make a bravery potion for the imp? (recipe can be found in the same dungeon)
Goblin trouble - I killed Lawrence too =/

Sorry but yes… with two un-spent skill points you can wait for 2 more and spend them to raise a skill from 3 to 4.

p.s. perhaps worth noting that the "all skilled up" talent gives two bonus skill points as well.

The stone is in the tomb on the steps behind the two knights. When a Star Stone activates it leaves behind an inert stone. The stone is in the tomb, all purple glowing, but I can't pick it up. See here. I had considered that, but I don't see that as a reasonable solution. The imps are/were slaves. I don't want the watch to take them back. If that's the only solution, then it stinks. Yes, but it's called 'Filter of the Fearless Imp', and he still won't talk to me. He just keeps crying that he's bad, he shouldn't have done it, and the master will kill him. No other dialog option. Lawrence got what he deserved at the hands of the townspeople before the mines were destroyed. Did that quest complete for you?

Did you try holding ALT and clicking on the text "Inert Stone"?

If you sent them away, you can find them in the beach next to the two drunk soldiers you met at the beginning.

Did you talk to the imp with the same char who had the potion in the inventory?

Yes it did....
I gave him the goblin head first, wich he rejected, he only died after that, and after the mine was destroyed it was registered as completed.

EDIT:

Oh, I see on your screenshot that the Watch is still there. Tell them you know where the imps are, then lie that they are at the beach.