I'm well aware of the anchor issue that occurs when too many addons are pulling from LibAddonMenu. I've dealt with it before and purposely slimmed by addon folder down by more than half. The list I provided you is the slimmed version, and I don't consider that a lot by a long stretch.
However, in the other case I've dealt with this error, it was completely benign and actually had no ill affect on any of my addons. That isn't the case here with Librarian, in which it's causing a massive string of errors and invalidating the settings panel. This I haven't seen before.
After testing, it seems to be directly linked to S'rendarr
I can reproduce the error every time with only Librarian and S'rendarr activated. With S'rendarr deactivated, but Librarian and the rest of my list enabled, there is no issue.
I could cross-post this error over on that addon's page if you like, as well. Due to how this particular anchor error is occurring with no variance (unlike when I was testing the benign type), I don't think it falls into the same vein as before and might be a different problem with either one or both of the addons in question.
But, I can't code, so what do I know? Hopefully this helps.