@Deantwo: Thanks for your answer. I think I was a bit confused, what's necessary and what is not. It works now. I had to repeat some steps over and over until it finally worked and the variables are changed. I'm not sure if that was my doing it wrong or if some specific action has to be taken, after...

This issue seem to have been addressed in RouterOS version 6.43, so it is much easier to fix these issues now. https://forum.mikrotik.com/viewtopic.php?f=21&t=138995 What's new in 6.43 (2018-Sep-06 12:44): /system script set [find name="Netwatch up script"] dont-require-permissions=ye...

4 and 1/2 year later: Same problem. Same question.

Solution:

File must have .rsc suffix!