Much of my research is with the Unindexed Historical Records in Sweden and Denmark. In the past I have been able to click on the button "Attach to Family Tree" but over the last week I get a page that says: "Well, this is unexpected. . . . We can help you search literally billions of records, but we can't seem to find the page you're looking for."
I've gotten this message when searching in various collections, but today I was searching the Kalls-Nobbelovs, Malmohus, Sweden 1797 Household Records.
I use this a lot so hope it can be repaired.
I was unable to find anything that I can browse from home from Sweden, Malmöhus, Källs-Nöbbelöv, and the "Attach to Family Tree" button worked just fine when I used it a bunch of times yesterday evening (on marriages in Hungarian church registers). Therefore I suspect that about a week ago, you had a momentary internet glitch that caused that error message, and your browser "helpfully" saved the corrupted file.
The easiest way to test whether I'm right is to temporarily try a different browser. If it works there, then you need to clear the saved history (cache and cookies) relating to FS in your original browser, and try again.
Thanks for the reminder. I cleared the cookies and now it works as before.