That was my first thought. Unfortunately, I can't find any forum for Swiffy on the Google site (Swiffy is made by Google). I've contacted them directly but I don't have a lot of hope I'll get a response.
You have to locate the Export as HTML 5 (Swiffy).jsfl.
When swiffy installs it will add this command somwhere on your harddisk. Probbaly under a folder that is named "commands" the exact location differs from OS to OS.
Look at line 26 and report back
As there is no good documentation about the swiffy internal workings (at least I found noting on googles site) at this point it becomes a wild goose chase. My first guess would be that it has to do with path-problems. If you are working on a network drive, try to change fla and htlm/js output to local storage, if you already compiling locally try a directory that is not nested deeply and were you are sure you have complete rights.
Yeah, I thought of that and moved the file to my desktop. It actually got a bit further before giving me the same error message. It got as far as creating an swf file. I installed Swiffy on an older version of Flash and it works on that provided the file size is below 1024 (I think) kb.
In the meantime though, it looks like Flash is dying a slow death and I'm working on gettng up to speed on Edge Animate to replace my Flash files. It would be nice to be able to simply convert my files, but learning a new program never hurts.