The “official” view on server software is this:
However some users report that Asset ( UPnP server software) works fine. So there seems to be some issue with your setup that differs from what others experience, However I do not quite understand what you are referring to regarding Plex as this seems to be another server software version or app mainly intended for A/V use :
So I do not understand quite where Asset in conjunction with Plex enters the picture. Maybe I have misunderstood your post but without further clarification the result you have would seem to be resulting from incompatibility as explained in the FAQ link.
I would simply download MinimServer 2 to your server hardware with MinimWatch 2 to your computer for control purposes and see if that fixes things after scanning your albums to it.