'Not a valid install package'

I recently posted a new Mozilla utility on mozdev.org: JS Console.

This utility has an installation trigger installing separatly two 
packages: mozlib package (a library of interfaces classes to XPCOM 
objects) and jsconsole package (the utility).

Testing the install on my computer (Windows and Linux OS) everything is 
OK. From the mozdev.org site, the installation sometimes succeeds, 
sometimes fails with the 'Not a valid install package' message.

However, both packages seem valid: they have their install.js script and 
their rdf files to register in chrome.

Does anyone had such an experience and have an explanation?

Daniel