system looses contact after installing network bridge

[robert]

Hi there

I wanted to install a network bridge on a remotely running ubuntu box 
running 14.01

there were allready two bridgies runnin, virbro and virbr2
so I added a third one executing:

brctl add virbr3 (ore something like that, I have no access to the 
computer just now)

after that the network froze (no contact to the host anymore)

I could log into a rescue system and did uninstall the bridge-utils.
After this I could access the boy again.
But when I reinstall the bridgeutils again, the contact breaks down again.

where should I look for the reason.

thanks for your help
robert