have same probelms ,
keepass are uploading a .tmp file tiil uncompete the file.
after uploading keepass checking the .tmp file and try to rename the file to origin file name( whatever.kdbx).
this renameing-process are chrashing in web.de/gmx.net webdav conection.
(read/write/rename rights , i think )
web.de/gmx.net throw the file-floder and the .tmp file into online-trash-folder
out fishing the .tmp file from the online trash folder and rename ist manualy in whatever.kdbx. that will work.
keepass2 webdav process are not compatible to web.de/gmx.net ( my experience)