MMW can't search a DLNA Server, so either you're searching the File List (list of files displayed) or you scanned the files into the Library. MMA can search on DLNA Servers and testing it against MM5 it worked to find Title, so perhaps the DLNA Server you're using doesn't provide this.

The NAS is a LenovoEMC, former Iomega, drive. Connection with MMW 4 and searching for tracks works properly.
With MMA Pro on Android 8 phone connection and searching for Albums and Artist work. Searching for Tracks failes.

It works here, what DLNA Server are you using? I tried with MM5 and it gave me the Title results.

My MMA is installed on a Samsung Galaxy smartphone and accesses music on an UPnP NAS via Wifi. Everything, so far, works fine except searching for tracks.
Searching for Albums - works.
Searchng for Artists - works.

Searching for Tracks - doesn't wok. Any suggestion what to do?

Thanks in advance.