@sand15 Thanks a lot. I forgot that this command exists. I'm a bit rusty with Maple. :)

1) Yes, I want to calculate quantile. Well spotted :). 

@Note2: Yes, with quantile it works fast and I might not need threads. However, I have a more complex script wich took me 16+ hours to compute. So I wanted to ad threads/Tasks and it didn't work (same error as above). SO I thought, I try to implement threads on an easier example. I did this but I got the same error message. Because it was easier to explain and understand, I posted the simple skript :).

However, my error is most likely caused by solve, int because they are not thread-safe.

@note3: Again, well spotted. My solve function retuned sometimes values like "ddd.dd + 0.0 I" (d = number), which is kinda pointless (that it is complex). However, because I wanted to solve the multi threadding problem first, I just simply used "with(RealDomain):" to avoid this problem while I was focusing on the threadding problem.

Thanks again, I will use the function Quantile.

@Carl Love Thanks for that. This is most likely the reason why it crahses or produces this error.
TBH, I didn't think that solve or int weren't thread safe, because those are omnipresent functions in Maple :).

Thank you!

@tomleslie thank you for suggesting fsolve(). I used Maple in my undergrads :), but now I'm a bit rusty and have to look up a lot. I have forgotten about fsolve.

However, I know that this problem is probably not worth doing, but the reason of my post was that the parallelising didn't work & I didn't understand why. 

If you use a Windows 10.2 with Maple 2016, the maple kernel crashes if you execute my code. So, your installation is probably fine :)

solve doesn'T return all values, because some of the results are complex like "xy.z + 0.0 I". I used "with(RealDomain):" to avoid that (was just a quick and dirty solution to a problem that I wanted to figure out as soon as everything works :).