dkms does not find kernel source

For days now I am trying to get ZFS-on-Linux working on Debian 7 but the module doesn't want to compile:

# dpkg-reconfigure zfs-dkms

------------------------------
Deleting module version: 0.6.3
completely from the DKMS tree.
------------------------------
Done.
Loading new zfs-0.6.3 DKMS files...
Building only for 2.6.32-openvz-042stab090.5-amd64
Module build for the currently running kernel was skipped since the
kernel source for this kernel does not seem to be installed.

Of course, headers and source are installed:

# uname -r
2.6.32-openvz-042stab090.5-amd64
# dpkg -l | grep 2.6.32-openvz-042stab090.5-amd64
ii  linux-headers-2.6.32-openvz-042stab090.5-amd64   1                                  amd64        Header files related to Linux kernel, specifically,
ii  linux-image-2.6.32-openvz-042stab090.5-amd64     1                                  amd64        Linux kernel binary image for version 2.6.32-openvz-042stab090.5-amd64
ii  linux-source-2.6.32-openvz-042stab090.5-amd64    1                                  all          Linux kernel source for version 2.6.32-openvz-042stab090.5-amd64
#

So I am trying it manually. Still not:

# dkms build zfs/0.6.3 -k `uname -r` --kernelsourcedir=/usr/src/linux-source-2.6.32-openvz-042stab090.5-amd64
Error! Your kernel headers for kernel 2.6.32-openvz-042stab090.5-amd64 cannot be found.
Please install the linux-headers-2.6.32-openvz-042stab090.5-amd64 package,
or use the --kernelsourcedir option to tell DKMS where it's located


ls /usr/src/linux-source-2.6.32-openvz-042stab090.5-amd64 | wc -l
90

it contains everything (arch, block, configs, debian, ...). WTF?

After a looong debug session I found that the build and source links in /lib/modules/2.6.32-openvz-042stab090.5-amd64 to /usr/src and /usr/include were missing.

Added them manually and it worked.

Everyime happy that I can work in Linux where I can at least look into the source and debug...