i filed a bug report and supplied a fix for the "problem".
since i get no answer via the mailinglist, perhaps it works here.
I have the same problem using an external webdav server (nextcloud).
Reading the docs and the source, the sabreclient 2.1.6 will be used.
It's quite old and nextcloud 12 uses sabreDav 3.x.
I can GET files but not upload then via roundcube files app or save to cloud.
A file will be generated, that's the first part (0 bytes) and then the file should be uploaded via PUT.
Perhaps Alexander can give me a hint how to enable debugging to get the headers sent to the server (content-length etc.)
For now i will try to create my own webdav module with debugging options.
Also , if a have time, i will strace all services and debug in depth.