error writing input to commandERROR: Invalid username and/or password
I got more information when I tried to rebuild another backup job in EM using the credentials. I got the same error when I tried to use the same Host Credentials I got the same Invalid username and/or password error on the page. 

I could not find anything authoritative on the subject but I read a list of prerequisites from this Oracle Community thread. Paraphrased..


  The user needs to be member of the ora_dba group and have local security privilege Log on as a batch job.


In my case the former was true but the latter was false. An admin on the team was "cleaning" up pcs in an OU and moved this server into another with different GPOs applied. 

So in the end, I gave the local account, used as Host Credentials, Log on as a batch job rights and the backup jobs continued to work again.