Catchable fatal error: session_regenerate_id(): Failed to create(read) session ID: user (path: /var/lib/php/session)
Thanks to bellosthomas, I was able to determine that our DB_Sessions class was causing the issue. It was a simple fix of FORCING the return values to be strings. I have marked the code and comments with BOLD and UNDERLINED text so you can see the strings. Just make sure that your "sess_read" method is returning a string!
[2015-11-13 05:23 UTC] bellosthomas at gmail dot com
I was also getting a fatal error (check comment above), with a custom session handler. After yohgaki's comment about the return value of the read() method, which needs to return a string, I checked my session handler and I saw I was returning boolean false in some cases. Casting the return value to string, solved it.