Core

I also (finally) found the problem that prevented the backend login. This time it's not PHP, but MySQL/MariaDB strict mode ;-)

class.t3lib_userauth.php getNewSessionRecord() does not set the ses_data field, which has no default in the database. The result was that $GLOBALS['TYPO3_DB']->exec_INSERTquery failed.

To make things worse, exec_INSERTquery fails silently. It should at least report the error to the devlog...

That all said, here's the patch (for 4.5.30, but it's probably needed for all Typo3 versions):

--- typo3_src-4.5.30/t3lib/class.t3lib_userauth.php       2013-09-12 11:30:04.000000000 +0200
+++ typo3_src/t3lib/class.t3lib_userauth.php    2013-09-29 15:51:45.000000000 +0200
@@ -813,10 +813,11 @@
        function getNewSessionRecord($tempuser) {
                return array(
                        'ses_id' => $this->id,
                        'ses_name' => $this->name,
                        'ses_iplock' => $tempuser['disableIPlock'] ? '[DISABLED]' : $this->ipLockClause_remoteIPNumber($this->lockIP),
                        'ses_hashlock' => $this->hashLockClause_getHashInt(),
                        'ses_userid' => $tempuser[$this->userid_column],
-                       'ses_tstamp' => $GLOBALS['EXEC_TIME']
+                       'ses_tstamp' => $GLOBALS['EXEC_TIME'],
+                       'ses_data' => '' ### cboltz - http://forge.typo3.org/issues/52358
                );
        }

TYPO3 do not support MySQL/MariaDB Strict mode ... there are more open issues about that

http://forge.typo3.org/issues/18866
http://forge.typo3.org/issues/18821
http://forge.typo3.org/issues/20052

yes, the issue should be solved, but there will be more errors afterwards. :-(