After installing the "HTML5 Swiffy Insert" plugin on my WordPress 3.8.1 site, I was having the same issues as others until I discovered that the plugin file "html5-swiffy-insert.php" was blank after the install. The fix that worked for me...
- Located the "html5-swiffy-insert.zip" on my hard drive.
- Unzipped the file.
- Opened the "html5-swiffy-insert.php" file into my text editor and copied all of the text.
- Went to my plugins in my Dashboard.
- Clicked the Edit link below the swiffy plugin.
- Clicked the "html5-swiffy-insert.php" plugin file.
- Pasted the text into the blank page.
- Changed the code 'https://www.gstatic.com/swiffy/v5.0/runtime.js' to 'https://www.gstatic.com/swiffy/v6.0/runtime.js'.
- Clicked the Update File Button.
Finally!! The swiffy animation plays just like the original swf.
FYI. I converted my swf using google swiffy and the swiffy extension for Flash pro. They both work for me. I hope this fix works for you.