Thunderbird 15 broke the zindus addon and the bug didn’t get noticed until too late.
The bug has been fixed in release 0.8.41 and later. If you’re using a version that you downloaded from http://zindus.com/i/download you can simply upgrade in the usual way (eg stop and restart Thunderbird).
If you’re using a version installed from addons.mozilla.org you’ll have to wait for their normal review processes to play out. I’ve asked for the review to be expedited but their editorial resources are stretched pretty thin. You’ll know that it’s been processed when you see version 0.8.42 listed at: https://addons.mozilla.org/en-US/thunderbird/addon/zindus/
For developers: if you’re interested to know what the problem was, the fix is on github. The breakage happened because XMLHttpRequest started respecting headers containing null values. Google quite reasonably began responding with http status code 406: No acceptable type available. The fix is to not pass headers containing null values to XMLHttpRequest - a silly thing to do in the first place.
Looking forward, it’s hard to promise that this won’t happen again.
Thunderbird recently changed its release cycle from twice a year to once every six weeks. It presents a huge challenge in terms of testing. Your humble developer can’t simply use the beta release of Thunderbird in day to day work because other critical addons (esp Lightning) aren’t always available in the beta release stream.
The Thunderbird team have decided (rightly or wrongly I don’t know) to trade improved security and stability that come with more frequent releases of the base product, at the cost of increased likelihood of breakage in addons.