Some of our users currently seem to experience problems with Lightning becoming incompatible when they update their Thunderbird installation to the 3.0.3 release.
The reason for this seems to lie in the fact that those users use a Lightning nightly build and update it on a daily basis with the Lightning Nightly Updater add-on or manually update to the latest available nightly build. But because we moved our Lightning nightly builds to only support the current Thunderbird 3.1 development builds, as reported earlier, this leads to the incompatibility.
The solution is simple though:
Just uninstall your current nightly build and install the Lightning 1.0 beta1 release. This release is identical to the latest available Lightning nightly builds for Thunderbird 3, but fully compatible with the Thunderbird 3 release. And to be on the safe side: Please backup your profile folder, before doing this!