Hello Angelika
It does sound like some action you are performing stops your ‘spasm’ from resolving. It could be the smallest thing too.
I think from your message that you must be right handed. If ironing and typing are setting it off it could mean that in order to operate you need to adjust yourself to accommodate the height of your ironing board or table. Often with the effort to of ironing one raises the shoulder a bit to drive more weight through the arm into the iron.
Maybe you can lower your ironing board or better still get someone else to do the ironing
. Do like I do, and simply don’t iron.
You may need to sit higher at your desk. I am not familiar with all the muscles you use to kayak (that’s a confession!) but that is definitely a more heavy-duty activity. What I do sense is that moving your shoulders down and back at every opportunity in yoga and in your normal everyday life is going to benefit you enormously. And of course with correct shoulder position you are practically forced to sit and stand with a straight spine. You may like to feel that your breastbone is also lifted which is another way to achieve your aims. Your neck will also lengthen through decompression of the muscles and spine.
Angelika, you may find you have some more questions that are pose-specific. Feel free to post them too. Most of shoulder, neck and upper back pain is going to be at least partly resolved with correct shoulder and arm movement.
Namaste
Gabrielle