I think it's a good idea to crate train, because there may be times when a dog has to be crated (eg after vet treatment). But I don't think keeping a dog in a cage for long periods is good.

We have a good guide to crate training here.

Emma Judson's guide to crate training

The following guide was written by the behaviourist Emma Judson and is shared with her permission. Crate Training - Step By Step Guide to A Distress Free, Force Free Crate Trained Dog or Pup Please sit and read this guide thoroughly before making a start, as it is important that every...

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Teaching your dog to be comfortable when left alone is a whole other matter though. Is that a problem for him?

 

Thanks for the crate training guide!

Yes that is a bit of a problem for him.. I didn't start when he was a puppy so it is my fault..
I am doing a program for separation anxiety now, so it takes a lot of time and goes slowly. I am not really sure he has separation anxiety (haven't noticed sweating, drewling, vomit pr such) but before I started the program, I actually left him a few times with success, but then after a while, if I left him 20 minutes, he would bark intense for 20 minutes... So I figured he got more and more stressed.

 

Yes, it sounds like 20 minutes is too much, too soon.

Incidentally, you won't see him sweat. Dogs sweat through their paws.