Just thought I would post my solution for this error as I have had a it a few times during my testing and live setup and it had me somewhat puzzled!
TL:DR: Check C:\Users\,username>\AppData\Local\Temp\2\McAfeeLogs\EPO510-Install-CustomAction for a line " Password has invalid characters in it." > change password to something less complex
My initial test setup was on a W2k8 vm using a local instance of sqlexpress. Initially this installed fine however something happened so I started again - a total reinstall.
This time around, I got stuck when trying to create the sql express database. I was getting an error "unable to connect using the information you provided" .
I then installed a full version of sql 2008 (locallay) yet I still got the same error.
In both instances I was using mixed auth with a windows user a/c.
Eventually I created a new SA a/c and managed to succesfully create the database using this sql a/c.
I have just gone to create the same in our live environment but using a separate existing SQL instance and a new install of ePO 5.1 on a W2k8 vm and I received the same error.
In all instances I was at first using my windows admin account. I then created a sql login, tested it was ok and tried that. Again it failed. Interestingly I had used the same password for both my sql and windows a/c.
I did a bit of digging and checked a log file:
This had a line " Password has invalid characters in it."
Both accounts had strong passwords using alphanumierc characters and special charachers such as *
Although this worked (and is supported by SQL) when logging into SQL Mgmt Studio, there's obviously some issue with the installer.
hope this helps someone