Package: python2.6 Version: 2.6.6~1 Severity: important I could now track down the source of the error.
It happens when installing other packages with setup-tools, that generate include files like the attached easy-install.path in /usr/local/lib/python2.6/dist-packages. Deleting easy-install.path makes class json.JSONDecoder work again.
easy-install.pth
Description: Binary data
signature.asc
Description: PGP signature
