External Storage/webdav - upload via webclient fails


I connected my HiDrive account as external storage via webdav. Everything works fine while accessing owncloud using "https://[domain]/remote.php/webdav/" from windows explorer: downloading, uploading, deleting, creating files. But when accessing the HiDrive-folder from the webclient, I can only download files. When trying to upload even small files, the process starts, showing the progress bar, but ends saying "not enough memory". Hovering over the upload button, it says "upload (max. 0 B)". All other folders work as normal (local, external/dropbox).