Weird unmet build dependencies on buildds

[Ole Streicher <olebole@debian.org>]

Hi,

I am trying to get my package "dpuser" compiled. While this works nicely
on my local pbuilder with up-to-date packages, it fails on the buildds:

https://buildd.debian.org/status/package.php?p=dpuser

f.e. for i386:

-----------------------8<-----------------------------
Dependency installability problem for dpuser on i386:

dpuser build-depends on:
- i386:libvtk6-dev
i386:libvtk6-dev depends on:
- i386:python-vtk6 (= 6.3.0+dfsg1-1)
i386:python-vtk6 depends on:
- i386:python-twisted
i386:python-twisted depends on:
- i386:python-twisted-core (>= 16.2.0-1)
i386:python-twisted-core depends on:
- i386:python-openssl
i386:python-openssl depends on:
- i386:python-cryptography (>= 1.3)
i386:python-cryptography depends on missing:
- i386:python-cffi-backend-api-min (<= 9729)
-----------------------8<-----------------------------

I could somehow not trace this dependency chain; locally everything
works well.

What could cause this problem and how should one solve it?

Best regards

Ole