Chain Skipping. Help appreciated!

You could also buy a 105 cassette, a bit cheaper than ultegra. I disagree that more expensive parts lasts longer concerning group sets. Dure Ace and Ultegra are made to be as light as possible. They may also be a bit more precisely made to secure smooth operation, but I do believe they wear out faster. Weight and durability seem to go well together if you ask me.

Perhaps your bike came with a cheap chain that has worn out the cassette faster? The chain had a problem with a pin which might also have contributed to some kind of imbalance between chain and cassette. . Did you buy a quality new chain at Decathlon? 

Wear and tear depends on many factors. My Bianchi didn't come with the best chain around. I rode about 4,500 km's and the cassette needed to be replaced (which was an original Veloce). I bought a KMC chain, well one of the cheapest, just replaced it after about 3,000 km's. 'Park tools said 0,5 but not too far from 0,75)'. This time no problems whatsoever with the cassette. Was it because I replaced it earlier or was it because I started to simply wipe my chain with wet wipes, or just a dry cloth, after every, almost every!, ride on this second chain? Nobody will ever know. 

Hope you are a happy rider by now!

Nice to hear thats sorted Hussavik. I don't know if you should have headset spacers rattling, they should be under a bit of pressure, Park Tools excellent website gives excellent advice on setting up headsets on https://www.parktool.com/blog/repair-help/threadless-headset-service  , parts 8 and 9 for this.

On one of the pages there is picture of a bike, and you hover over the bits you want to sort.  Their method for fine tuning cup and cone wheel bearings made it so much easier for me.

And theres always Sheldon Brown if you want to know everything.

Chain snapping, had a similar thing with a pedal axle snap, unpleasant, sympathy    ...  

I think, it does sound like the cassette might be worn. However, one other thing that might be worth a shot is to check the B-Post tension is correct. Putting a new chain on could well have upset that setting and if there is not enough chain wrapped around the cassette it will be more likely to shift badly and slip under load.

The danger changing just the chain is skipping, as has been said much of the skipping is on 2 or 3 sprockets, the most often used ones. Ultegra/Dura Ace are sets where when I change chain/Cassette combo to save customers returning....and .....often if you run your fingers gently over the sprockets you'll feel "sharp" edges, rather than "squared" ... I've even seen teeth which resemble shark fins...... The only grumble I get is the cost of changing cassette/chain combo, I just have to explain the components are lighter and often a softer material...... Wait until you have to explain a SRAM Red single block cassette needs changing.... ugh! ....

I use Park Tool recommended chain replacement rule:

10 speed or lower system replace chain @ 0.75%

11  speed replace chain @ 0.50%

Could be a worn cassette and if it only does it on a couple of sprockets (the ones you probably use the most) it is most likely down to that.

It could however, also be due to a sticky or stiff link in the chain, which is an irritatingly common issue with new chains (about 5% IME)

To diagnose this, put the bike in the workstand and turn the cranks in reverse. The chain will skip or click at the rear mech & cassette and/or the chainring when the stiff link enters that section of its revolution.

It does sound like a worn cassette / sprockets, but I disagree that 2000m is enough to wear out a cassette / sprocket. Most of my cassettes last 6-10 chains which is about 30,000km before any issue of skipping begins. However, if the derailleur is straight, and the pivot pins aren't bent (if there is damage in the elbows of the derailleur, it can go out of line as it moves along the cassette), and the hanger is straight, and the gears have been tuned from zero, then worn sprockets is pretty much the oonly possible issue left.

Sounds like a worn cassette to me. Skipping on just the most used sprockets is definitely a symptom of new chain and old cassette.

Get the cassette changed and keep the old one in case there's no difference (i.e. the old cassette wasn't too worn). Another possibility is if the cassette hasn't been tightened sufficiently, though that would most likely cause issues with changing gears as well.

2000m or 3200km is easily enough to wear out sprockets. It is perfectly normal for just one or two sprockets in a cassette to wear significantly more than the rest and what you have described indicates to me that you have been using 3 & 4 ( a lot) more than the others. There is one other aspect here and that is the quality of the cassette. Generally speaking, the higher the price (better groupset) the better the quality and the higher the mileage. You wrote that the bike is 2000m old so I understand that the cassette is the the one that came with the bike. A common trick manufacturers like to use on new bikes is to use higher value components that a buyer can easily see and compensate with lower value components, like a cassette, that are not so easily identified.