I have the same problem.
Maybe this "bug" (?) will be fixed soon since backup is still a beta.
I should mention that there is only one MSQL running on my system.
Maybe someone has a solution to fix this error
Edit:
Oh I found something Kai wrote in another post. I'm a stupid one. I should have read the whole new posts before writing something
Kai Seidler wrote:Hmm... _ (underscores) are no problem.
Ahhh...
stupid me!!!! You found a bug!

Again... please take your favourite editor and edit
/opt/lampp/lampp.
At line 400 you find:
/opt/lampp/share/lampp/backupPlease add a space and $2 to the end of this line. Like this:
/opt/lampp/share/lampp/backup $2And now try again. Now it should work perfectly.

Repute, thanks for "finding" this bug!!!
Greetings
Oswald
Have to test this maybe the problem will be fixed then.
Here is the link to the post to read through:
http://community.apachefriends.org/f/viewtopic.php?t=4063