In a nutshell:
I use an apache2 as webdav server and have a nginx reverse proxy in front of it.
The download of larger files (~ 6 GB) works without problems only the upload of larger files (already from 450 MB) aborts exactly at the EOF with the error message "65 client intended to send too large body " (on nginx).
I tested all possible settings like client_max_body_size, proxy_buffering off; unfortunately without success. If I run apache2 without nginx reverse proxy everything works as expected.
Thanks for your help