Many of you have already mentioned this, thank you for the feedback! This is obviously an important issue so we will release an update that fixes it. I will upload it to addons.mozilla.org within the next 12 hours, then it only needs to go through their review.
Users depending on inviting attendees are advised to downgrade to Lightning 1.5.0 until the Lightning 1.5.2 release is live.
Also, on a side note, we finally have a new Mac OSX nightly for Thunderbird Daily! You can get it on ftp.mozilla.org, as always. Windows is so close to being ready. There was another IT issue which is fixed now. Now another build automation issue showed up, which is hopefully the last one.