backup exec error connectig network

We have backup exec 2010 in a network and the server we want to backup in other one. When I try to backup a folder in the other network it ask me a username and password, until here its fine. When I set the username and password not as "default logon" it doesnt work, when I set it as default and change the username and password of my account to the network I want to backup it works fine. But It has to be set as "default logon", and so all my other backup sets dont work becouse I changed the username.

Was it clear? How can I fix it??

Thanks.