error CS0117: System.Collections.ObjectModel.Collection> does not contain a definition for Items

We are having this error only on certain machines, and then only once in a while:

error CS0117: 'System.Collections.ObjectModel.Collection<Node<T>>' does not contain a definition for 'Items'

Since it's part of the system library, the only thing I can think of is that there are competing .NET libraries and somehow Unity decided to compile with a different one. That is worrisome for me -- hopefully it's not that case.

We do have multiple versions of Unity installed (all Unity5+, pre Unity5 versions were uninstalled).

Any idea what would have caused it? Usually a force sync/pull of the whole folder fixes it but that is a pretty heavy weight solution and it's happening ~1x/week.