Sometimes you are not able to backup the exchange databases because you get an error regarding to the writter, here you have the information how to fix it.
Open a command prompt as an admin
To see what writer is failing please type:
vssadmin list writers
Open a command prompt as an admin and write the follow line:
NET STOP MSExchangeRepl
Once the service is stopped:
NET Start MSExchangeRepl
if its Microsoft Exchange Writer:
net stop MSExchangeIS
net stop VSS
net stop MSExchangeRepl
net start MSExchangeIS
net start VSS
net start MSExchangeRepl
And voila, tray again with your backup script and should work.