My fstab contains CIFS-paths which have been working in previous releases for thunar:

$ cat /etc/fstab
#works not in thunar, but mounts on file system
//server1/path1/to /path/to/local/mount1 cifs _netdev,credentials=/home/user/.smbcredentials/file,iocharset=utf8,users,noauto 0 0
//server3/path/to /path/to/local/mount4 cifs _netdev,credentials=/home/user/.smbcredentials/file,iocharset=utf8,users,noauto,nounix,sec=ntlm 0 0

#works in thunar, and mounts on file system
//server1/path2 /path/to/local/mount2 cifs _netdev,credentials=/home/user/.smbcredentials/file,iocharset=utf8,users,noauto 0 0
//server2/path/ /path/to/local/mount3 cifs _netdev,credentials=/home/user/.smbcredentials/file,iocharset=utf8,users,noauto,nounix,sec=ntlm 0 0

$ thunar --version
Thunar 1.6.10 (Xfce 4.12)

$ thunar-volman --version
thunar-volman 0.8.1 (Xfce 4.12)

So what is not working:
clicking in Thunar under section "Devices" to mount the paths the latter two mount points are mounted appropriate (in file system and in thunar), the first two do not work. Well, all path are actually mounted and accessible on the file system, but Thunar does not recognize that for the first two "Devices". I get the waiting icon, and clicking twice on the same "Device" tells me that the resource is busy. The only difference that I see is the remote path length.

The following command works without any issues successful.
$ mount.cifs -vvv -o _netdev,credentials=/home/user/.smbcredentials/file,iocharset=utf8,users,noauto //server1/path1/to /path/to/local/mount1
domain=xy