I administer Atlassian stack instance (among zillion other things, of course). Once I've got an issue about login problem:
"I can login to Confluence, but not to Jira, could you help me?"

Looking into projects configuration, into user's permission groups in Crowd (both apps are connected, it will be important in a moment)... Everything looks good. Wtf?

Suddenly, I've got this idea:
"What username do you use in Jira?"
"My username."
"What about Confluence login?"
"My email."

Realization in 3... 2... 1...

Wait for it...

Just a little more tension...

"Nevermind, thank you!"

Remember, guys, always give them a chance. Plan for the worst, but hope for the best. And I wish you all only such issues! :-D