Never fear, Jay is here!
Ok, enough joking around.
Lets see what's up.
Go the the 'pixelpost.lrdevplugin' folder.
Open up the file PixelpostAPI.lua.
One line #40 you should see this code:
Remove the two '--' at the beginning of the line.
And save the changes.
Now restart Lightroom and attempt to upload an image to Pixelpost.
The Plugin will now create a log file which you can look at to see the actual error message that occurred.
This log file is written to your ~/Documents folder in Mac OS, or to your My Documents folder in Windows.
Now when you get the error, open up the log file, scroll down to the bottom, and copy the complete error message into a forum post or PM so I can check it out.