I'm trying to work through this myself but have reached a dead-end.

After up grading an existing site to 2.4.0 I can't login to the CMS.

So - I've successfully upgraded but still have the original problem!!

I've gone into mysite/-config.php file and checked I'm using the same default admin login. Thanks Banal, I've redone this but still

when I append admin to the URL I end up with a cms-styled page saying (impenetrably unhelpfully):

Website Error
There has been an error

The website server has not been able to respond to your request.

Please help, this is the second day I've spent trying to access this CMS.

Thanks Banal, I've just tried it but it just goes straight to the front-end/homepage.

Do you reckon it's something to do with javascript with the change to 2.4.0.?

Thanks banal

the config file already has this in it -

Director::set_dev_servers(array(
   'localhost',
   '127.0.0.1',
));

But I figure that's no problem and added suggested code.

Appending admin now gives me a big page of errors:

[User Error] Bad RecordClassName '' and $baseClass not set
GET /admin

Line 2705 in /usr/local/www/vhosts/*sitename*.co.nz/httpdocs/sapphire/core/model/DataObject.php

Source

2696       foreach($records as $record) {
2697          if(empty($record['RecordClassName'])) {
2698             $record['RecordClassName'] = $record['ClassName'];
2699          }
2700          if(class_exists($record['RecordClassName'])) {
2701             $results[] = new $record['RecordClassName']($record);
2702          } else {
2703             if(!$baseClass) {
2704                user_error("Bad RecordClassName '{$record['RecordClassName']}' and "
2705                   . "\$baseClass not set", E_USER_ERROR);
2706             } else if(!is_string($baseClass) || !class_exists($baseClass)) {
2707                user_error("Bad RecordClassName '{$record['RecordClassName']}' and bad "
2708                   . "\$baseClass '$baseClass not set", E_USER_ERROR);
2709             }
2710             $results[] = new $baseClass($record);
2711          }

Trace

Bad RecordClassName '' and $baseClass not set
Line 2705 of DataObject.php
DataObject->buildDataObjectSet(MySQLQuery)
Line 569 of Hierarchy.php
Hierarchy->liveChildren(1,1)

etc..... there's half a page of these ...

That's a more fulsome error output but, for me, no more meaningful - although I do comprehend "user error" !

Anyone out there who can interpret this and assist me to get it back on track?

My reading today has resulted in a note from the SS datamodel page ( http://doc.silverstripe.org/datamodel?s=static%2520defaults ):

Generating the database-schema

The Silverstripe database-schema is generated automatically by visiting the URL.

http://<mysite>/dev/build

Note: You need to be logged in as an administrator to perform this command.

If I can't login how is the database being rebuilt? Is this the root of my problem?