Hi,
That is very strange. Whats happening there is that mojoPortal is using a tool called SubSonic to check for the existence of the mp_Sites table, this is how it determines if this is a new installation or an upgrade. SubSonic "should" be using the same connection string that already passed the connection test, but it apparently is not since it is complaining about the string not being in the correct format.
What I would try is putting the same connection string value as you have in this setting:
<add key="MSSQLConnectionString" value="server=(local);UID=mojouser;PWD=mojo123;database=mojoportal" />
into this setting at the bottom of Web.config in the connectionStrings section:
<add name="Subsonic-MSSqlServer" connectionString="server=(local);UID=mojouser;PWD=mojo123;database=mojoportal" />
maybe that will solve it.
Hope it helps,
Joe