Connection minimserver - UPnP control point unstable - issue NAS/Minimserver?

[mjolink]

I bought a Linn Akurate DS and a NAS (Synology 214Play) to store my music two weeks ago. From my UPnP control point (Linn Kazoo and BubbleDS Next) I can see my Linn Akurate.
The problem is that the connection UPnP control point to minimserver is unstable, most of the times I can't see minimserver from my UPnP control point, therefore can't reach my music. Sometimes the connection is there, but can't find what is causing it. When there is a connection and I turn off and then on wifi connection on my phone, the connection is gone again. After some time it is there again spontaneously. Although minimwatch on my laptop is green and there is a connection between the two, it can happen that I still don't see minimserver in Linn Kazoo/BubbleDS from my phone or Ipad.
I have already spent over 20 hours analysing and testing scenario's. The issue is that the problem isn't there all the time (not a black or white situation), but seemingly random.

What I have done so far:
- checked rights on minim folder and music folder on my NAS.
- completely removed Java (7 and 8) and minimserver and did a reinstall, installed Java8 package and Minimserver (MinimServer-0.8.5.3-intel.spk).
- unplugged router and NAS and put them back online after some minutes
- removed the '_' from my NAS name and station name thinking this might be the connection issue

I have isolated the issue to the NAS/minimserver OR connection NAS-Router. I installed minimserver on my laptop and tested it. This worked flawlessly! This is indicating to me that the issue might be in/around the NAS (or connection NAS-Router)

I can hand over log files. I have a screenshot of the connection to minimserver from BubbleDS when it is working available.

Who can help me in the right direction?