Hi Gabriele
Have you tried a Webdav client in Windows (other than the integrated one) ?
Mountain Duck allows you to mount and map Webdav-locations (and lots of other “cloud storage”) in the Windows explorer. Then you can open, save and close files as if they were accessed over smb (I never had to deal with files in the 300-400 mb range though). You can test it 30 days for free (€ 35 per user afterwards).
Raidrive is a similar product, which is free upon to 8 mounted locations (iirw).
With both clients, it is also possible to mount a folder shared in EGroupware without beeing a EGroupware user. You can mount “shared links” (password protected in the EPL-version of EGroupware, if needed). This is very convenient with external users.