One specific user has been experiencing difficulties logging in to my site. He's getting the following error: session_start(): Key cannot be empty.
I have no idea what this error means, and haven't been able to identify why this happens only for a specific user. My session saving is handled through memcache.
What does the error mean? What can cause it only in a very specific case?
This can often mean you're outputting something before the session_start, which shouldn't happen.
It could be a third party extension interfering with your page loading, I'd recommend the usual steps of use an incognito / private session, to see if problem persists. If not, it's a problem with his setup.
EDIT: However, this appears to be an existing bug...