I found the problem. As of r76780 the default for the TarFile.errorlevel argument changed from 0 (suppress errors and write them to the debug log instead) to 1 (raise exceptions for fatal extraction errors). This change was not backported to the 2.6 branch back then (it was blocked in r76781).

This means, that Python 2.6 does not succeed either, but the error is simply suppressed.

Ergo, this is no regression but a simple filesystem issue.