We were hit by this too. Upgrading the server to Jessie (Samba 2:4.2.10+dfsg-0+deb8u2) solved it, without any configuration change.
- Bug#820982: samba: Users cannot open their Windows... Sébastien Ducoulombier
- Bug#820982: samba: Users cannot open their Wi... Günter Frenz
- Bug#820982: samba: Users cannot open thei... Andrew Bartlett
- Bug#820982: samba: Users cannot open ... Günter Frenz
- Bug#820982: samba: Users cannot open their Wi... Rune Tendal Kock
- Bug#820982: samba: Users cannot open their Wi... Vladislav Kurz
