If you got a shared-link to nextcloud, there you may upload files by drag & drop. Unfortunatelly it doesn’t work if you drop on lower area (the empty area below the last file). If you drop on file-list (usually upper half) then upload works.
There is no log on serverside if it didn’t work.
Affected version 16.0.3. Tested with Firefox 68.0.1 (64-Bit) on Windows 10.