Don't you just hate it when software develops a mind of its own such that it acts petulantly? Bad Sigil; no doughnut!

I am going to try and use telepathy to guess the names of the files you were trying to rename. Let's see; how about w00t.htm to w00t.xhtml ? No? Hmm... abc.html to xyz.xhtml ?

I give up. What were the file names (and what you wanted to name them) that failed and what was one of the ones that succeeded?