How important is specialisation in a master's degree? My university offers an optional specialisation (e.g. algorithms, software engineering, networks...) during my masters program (which will appear on the degree). To qualify, you basically need a certain amount of credits on that topic.

I think I rather not specialize, so I can hear a broader spectrum of topics.
How important is such a specialisation for future job search (in academia and industry)? Does it matter at all?
I can just answer your question from the industry side:

I know enough people that don't even work in the field where they studied. Here in my department (in a well known company) we have guys from computer science, engineering and electrical engineering working on the same tasks. It's more important to have a technical degree than have a good specialization. Most of the hard skills you need for your work aren't taught in university. The important skills are the soft skills that the job will give you (to complete in stem fields with good grades will show them that you are not totally dumb and have the stamina for hard tasks).

Visit the courses that sound useful to you. I did it that way and am completely happy with my decision.