Ran into this interesting scenario recently in my SCOM 2012 management group.
We use a specified account as the default action account for our SCOM management servers as opposed to Local System. Local System account is used for all other monitored servers.
Someone had changed the default action account for one of the management servers to Local System but when I tried to change it back to our specified account via Administration, Run As Configuration, Profiles, Default Action Account I got an error saying that the account that I was trying to assign to the server had a blank password!
This was strange because all the other management servers were already using the specified account!
I went into the SCOM console – Administration pane – Run As Configuration -> Accounts and found the Action Account we were assigning. Opened the properties and typed the password again. After doing this I could change the Default Action Account Run As Profile.