In ../blogs/inc/session/model/_session.class.php lines 153-171:
This code is going to throw notices if ini_set() is disabled. The if statement comes *after* the first attempt to ini_set():

					else
					{ // Some session data has been previsouly stored:

						// Unserialize session data (using an own callback that should provide class definitions):
						$old_callback = ini_set( 'unserialize_callback_func', 'session_unserialize_callback' );
						if( $old_callback === false )
						{ // this can fail, if "ini_set" has been disabled for security reasons.. :/
							// Brutally load add classes that we might need:
 							session_unserialize_load_all_classes();
						}
						// TODO: dh> This can fail, if there are special chars in sess_data:
						//       It will be encoded in $evo_charset _after_ "SET NAMES", but
						//       get retrieved here, _before_ any "SET NAMES" (if $db_config['connection_charset'] is not set (default))!
						$this->_data = @unserialize($row->sess_data);

						if( $old_callback !== false )
						{	// Restore the old callback if we changed it:
							ini_set( 'unserialize_callback_func', $old_callback );
						}

Error in:

$old_callback = ini_set( 'unserialize_callback_func', 'session_unserialize_callback' );

and in:

ini_set( 'unserialize_callback_func', $old_callback );

Reported by filthio in [url=http://forums.b2evolution.net/viewtopic.php?t=13308&start=0&postdays=0&postorder=asc&highlight=][2.x] 2.1.0 installation: ini_set() problems[/url]

Good luck