*Solution from Manage Engine Support*
"Please go to the installation directory of ServiceDesk Plus C:\ManageEngine\ServiceDesk\fileattachments. Under this folder create an empty text document. Try running the backup again.
where c is the installation drive where ServiceDesk Plus is installed."
I created a text file called "text" and placed it in the appropriate directory. The install worked fine after that.
Hope this may help someone in the future with a similar problem.