It definitely sounds like a strained relationship with Josh. Josh mentions its not football related, issues with "teamwork" but that "he (Josh) has to be better." Maybe suggesting Diggs has an issue with him, either his play or maybe Josh's personal issues last season which stef may feel got in the way.
Regardless of the issue, I can't imagine a situation where stef missing camp four months after the season ended is justified. He's a diva and drama queen and it's annoying.