I tried Oshyans suggestion, but had the same result.
In all the Poolhouse - projects I have in fact a warning of a missing .mtl but it's not really missing... and no problems with rendering.... strange!
I fear that project has grown too big, to check all the names of the few hundreds of objects and textures. I made the experience, that I must mostly reconnect every texture and in some cases completely "re-insert" the whole object. Only the repair-actions of the bike was several hours for example. In that time until i have checked and renamed everything in that project I would also have finished the renderjob...
But for future projects I know better now, what details I have to consider.
I have a perhaps strange structure of my project in your eyes: Theoretical in that procect are all objects and populations integrated so that I could render a 360° view. For my different views and animations I just deacvtivate the not needed populations and especially not needed reflective objects to spare rendertime.
Anyway for the moment I must replace the old Acer. I checked the hardware and must correct my last info: Its one of the last AMD Phenom II 4x2.8 GHz / 8GB Ram / Redeon 65xx Graphics. It was one of the best in 2006 but must get buried now

It's even louder as my vacuum cleaner

I ordered a worthy successor I think (I have a coupon and a little discount therefore, it is an acceptable investment)
This will be the machine for just developing scenes while the two Asus do their render jobs. And of course I need it for my usual PC usage like homeoffice, internet and some graphics:
Prozessor: Intel® Core™ i7-4790 Quad-Core 4. Generation (Haswell)4C/8T / 3,6 GHz (Turbo 4,00GHz) / L3-Cache: 8MB / 84W
Motherboard: Gigabyte GA-H81M-D2V mATX
RAM: 8192 MB 1600 MHz DDR3 RAM
HDD 1: 120 GB SATA-3 SSD
HDD 2: 1000 GB SATA-3 (7200U/min.)
Laufwerke: DVD-Brenner
Grafik: Nvidia GeForce GTX760 Overclocked Grafik, 2 GB GDDR5 (2x DVI, Display Port, HDMI,) PCI-Express 3.0, unterstützt Nvidia GPU-Boost, Nvidia Adaptive Vertical-Sync, Nvidia 3D-Vision Ready, Nvidia Surround, Nvidia SLI®-Technology, Nvidia Physix® Technology