Would you happen to be using several different browsers at the same time?
I do it all of the time. I like to use Firefox with SpanishDict.com, but it is not my default browser. So if I look up a word in the dictionary or google info for a reply, it opens that window in my default browser. When you start jumping around from browser to browser, tab to tab, you can get logged out unintentionally. Then if you try returning to a SpanishDict.com (especially if you do it from the browser that you didn't start from) it tells you that it can't find the page (because you are no longer logged in to access it). Just a possibility.
I haven't been having your problem lately so I don't think that the SpanishDict.com server is acting up as it frequently does.