I logged into one site and ran it as a Joomla update and it works fine.
Which means that you have proven that the problem is
not on our side of providing updates :) We have not changed anything regarding the updates (yet). In the next version we'll move to full HTTPS for the entire update provisioning, including the update information XML. We do not foresee any issues. We've been trialing HTTPS on our CDN for 15 months to make sure that we have a robust process.
In any case, your current issue looks like something in Watchful.li. I'm pretty sure they'll look into it since you've already filed a support ticket with them.
May I kindly request that you give us an update when they reply back to you?
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!