ServiceM8 annually sees full "version" update right around this time. My guess is their developers are hard at work and are updating aspects of the product in preparation for impending updates. Since it still works in Chrome (and that's all they care about), this might be why it stopped working in Safari. Additionally, Apple might have changed something in a recent update that broke how that aspect of ServiceM8 operated; it was able to work before, but now not because of something that Apple updated.
Either way, for the best and most consistent experience, only work on ServiceM8 via Chrome.