Problem is that the problem is not always there. I have one (small) course where the problem has never shown up.
Right now my hypothesis is that it is a JavaScript error somehow. So I think one of the activities or resources I added tot my courses (easily hundreds for one course) calls a certain JavaScript component that **sometimes** leads to this error... vague, isn't it?
It would be nice to have an option to always show these with a button next to it. Yesterday I found another workaround. I have a browser plugin to enable/disable JavaScript with one click. So if I want to add a new component and it doesn't work, I turn off JavaScript, reload the page, and the activity/resource list appears with an extra button that always works. I fill out the necessary fields, and turn JavaScript back on before I submit the component to the course. It is a bit of a pain, but easier than my first hack.