We (Agro-Wodzisław) are using "Symfonia Handel" app. made by Sage. Program runs on Windows XP, files are stored on Gentoo Linux (kernel 4.5), served with Samba-4. After upgrade from samba-4.2.11 to .12 we could not login into Symfonia on more than one workstation.
Symfonia uses Btrieve microkernel running on every workstation. Switching back to 4.2.11 (compiled on the same system! [same options, packages, tools...]) made Symfonia run as usual (making .slck file for every user/workstation, while 4.2.12 made it only for the first user and next cannot login into application; without any warning - just the same login window and no .slck file saved on server).
I read all ~2000 lines of differences between .11 and .12, but hadn't found anything about locking.
We need the smb.conf and the following for against 4.2.11 and 4.2.12:
network captures and level 10 log files
Please start the capture before starting smbd (and if used winbindd).
(In reply to Stefan Metzmacher from comment #1)
I will test it of course (I have to prepare 2 VMs for that purpose first) but I recall now that I've tried samba-4.4.2 and than 4.3.8 (3 weeks ago) because I noticed the same effect! (only first user could login into database)