Snapshot Pro won't complete backup

I have installed Snapshot Pro on three of my wp sites. The first two backed up beautifully. The last one does not. I set it up exactly the same as the others, using dropbox, created the snapshot just fine. When I run the backup for the first time it runs through the list of tasks but when it gets to the 'Files' task it hangs. Is there some kind of limit on backups I can use with dropbox?
This is the error message:
"Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at to inform them of the time this error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request."
What gives?