drivername should be checked when creating the notifications table in updatedb_helper.php, line #1420 and following ( if ($iOldDBVersion < 259) etc....)

Just removing "default" from first_read column definition did the trick (if no value specified, null is default anyway, and column must be nullable):
if ($iOldDBVersion < 259) {
$oDB->createCommand()->createTable('{{notifications}}', array(
'id' => 'pk',
'entity' => 'string(15) not null',
'entity_id' => 'int not null',
'title' => 'string not null', // varchar(255) in postgres
'message' => 'text not null',
'status' => 'string(15) default \'new\'',
'importance' => 'int default 1',
'display_class' => 'string(31) default \'default\'',
'created' => 'datetime not null',
'first_read' => 'datetime null'
));

Thanks, will apply this change.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=20327

Version 2.51.1 Build 160901 released