Backup not working only on 1 domain

[cibertec]

I have 4 domians setup atm. I can do a backup on 3 out of the 4. When I try and do a Plesk backup I get the following error:

An unhandled error occured. The process has been stopped.
Details = Type mismatch in expression

Thats all I get. What would be causing this for only 1 domain but not the others. Anyone else having this problem. I'm running the Plesk backup from outside of Plesk so I can backup up all domains at once.

Thanks in advance.