i don't know if it's the EXACT SAME THING... but a similar thing happened to me just today.
are you using allembic? maybe for the bones?
this has happened to me before but not often so it hasn’t sinked in for me yet
i changed the name of my allembic files last night... went to sleep... and earlier today when i opened up the latest file that was using them... zsolver was there but no ztets and everything with a bunch of errors blah blah blah
BUT BUT BUT BUT BUT -- even if that may not be the EXACT cause of your issue... in fact it won't really matter what the issue is for the most part....
if you don't already (cuz i can't tell from this post or your post history), it's always good to take the time to learn how to script everything out so you can rebuild your setup easily no problemo just in case something like this or anything happens, because you're bound to come up with unexpected issues along your journey
but that's just my two cents; it helps me not be afraid of making a buttload of booboos. if it helps you, sweet (and i'm sure there's other ways to do it too), if not... i hope you solve it. if another suggestion comes in mind on why you're encountering this issue, i'll add to it
(and IF IT IS just a matter of allembic filenames being changed... just changing the names back should solve it, including for your backup files that were depending on it)