I've seen this situation as the following scenario:
- You have an enabled "sa" which' s password is Pa$$w0rd
- You create a maintanence plan using this Login using Pa$$w0rd
- You change your "sa" Login password as PassWord
- You forget updating your maintanence plan' s "sa" password as PassWord so it fails.
Ensure your "sa" password is correct.
Regarding to the invisible password, after clicking the New button and creating a new connection how can you see that window again? There is no any button there to see the connection you created previously. There is only New button there.