Instead of adding the source, I get what looks like a data page of some sort with spinning blue boxes for each item.
It looks like Source Linker didn't load fully. Have you tried simply refreshing/reloading the page? If that doesn't help, I suggest trying in a different browser. If it works there, then some "housekeeping" in the original browser may fix the problem: make sure the browser is up to date, and clear your FS-related browsing history (cache and cookies).
I see that occasionally. A cache clear always cleans it up.