Did some additional tests. Suspected it had something to do with WP Super cache. However, that’s not necessarily the case as this issue doesn’t occur on a domain I use for development, and has the cache installed (and activated).
However, the development domain is a single-site installation, whereas the live domain is running on a multi-site installation. Possibly, de-referencing the lib functions in the latter case has something to do with it.
Haven’t tested what speed advantage (if any) the google libraries plugin achieves in a cached environment. I suspect it isn’t huge. For the time being I’m fine keeping it switched off. Would be happy if there’s a solution that would allow reactivating it.
