Are you using Yatse as well, because some of the JSON requests in your log look like the polling spam typical of Yatse. If you are using Yatse, please disable it while testing with your Tuefel, as they could be interacting with each depending on how badly the apps have been written. If you are not using Yatse then maybe the Tuefel app has the same design inefficiency as Yatse.
In your kodi.old.log I can see (once I filter out the spam) that an app - presumably the Tuefel, maybe Yatse - is requesting song details in batches of 975 songs. It manages to request 16750 songs, then stops at 09:18:04.581- there are no further JSON requests (other than spam) which suggests the Tuefel is still working. Kodi doesn't appear to have crashed at this point as it is still processing the spam requests.
Someone (Teufel? Yatse?) starts a music library scan at 09:20:21.511, which runs for several hours, finishing at 14:37:30.579.
Then, at 16:30:24.536, there is a UPnP request (presumably from the Tuefel?) which seems to be requesting all songs - 49621 in total, in batches of 100. Kodi seems to responsd OK with the first 400 items, but then hangs when being asked to process the 5th batch. My guess is there is a serious memory leak (or scale issue) in Kodi's UPnP implementation, which is a highly experimental feature in Kodi 18.
Please open a trac ticket on Kodi - TRAC and include the link to kodi.old.log - this will need the Kodi UPnP developer to investigate further.