i had this issue too in latest staging using php sessions, not sure how it appeared.

@mbabker i can reproduce an issue in latest staging

1. Set the session handler to PHP (files handler)
2. Set session timeout to 1 minute (for faster test)
3. Login in backend
4. Wait around a minute
5. Refresh the page

Warning: session_start(): The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in /path/to/joomla/staging/libraries/joomla/session/handler/native.php on line 260

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /path/to/joomla/staging/libraries/joomla/session/handler/native.php:260) in /path/to/joomla/staging/libraries/joomla/session/handler/native.php on line 260

Warning: session_regenerate_id(): Cannot regenerate session id - headers already sent in /path/to/joomla/staging/libraries/joomla/session/handler/native.php on line 151

Warning: session_write_close(): The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in /path/to/joomla/staging/libraries/joomla/session/handler/native.php on line 154

Warning: session_write_close(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/path/to/joomla/staging/tmp) in /path/to/joomla/staging/libraries/joomla/session/handler/native.php on line 154

Error displaying the error page: Application Instantiation Error: Failed to start the session because headers have already been sent by "/path/to/joomla/staging/libraries/joomla/session/handler/native.php" at line 260.

Could this be related to the cookies PR #12306 ?

@mbabker please test #16481

Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/16477

closed as having PR #16481