After upgrading from 2.3.3 to 2.4.0 the following is shown:

Warning: Unexpected character in input: '\' (ASCII=92) state=1 in /home/trainer/public_html/sapphire/core/SSViewer.php on line 304

This appeared after a rebuild was carried out. Now it is not possible to do another, as attempts to login fail.

Can anyone suggest the fix for this? The relevant block is:

		$content = ereg_replace('<' . '% +_t\((\'([^\.\']*)\'|"([^\."]*)")(([^)]|\)[^ ]|\) +[^% ])*)\) +%' . '>', '<?= _t(\''. $identifier . '.ss' . '.\\2\\3\'\\4) ?>', $content);

Try adding your current domain to the list of dev servers in your _config.php file. That way you can see what error message you get on a rebuild without needing to log in.

Thanks Bruce, have done so and see in red:

user_error('DataObjectDecorator::extraDBFields() is deprecated. Please use extraStatics() instead.', E_USER_NOTICE);

To be honest, an upgrade shouldn't be this difficult. We have also tried a fresh install on a test server, then imported the database, which worked fine. However upon adding Blog, the User Form, and Mollum modules the same error arises. We've also asked this question a couple of times in the IRC channel and looked through the forums and help section, but it all appears support is targeted to advanced php developers only.

Had the exact same issue. Upgrade from 2.33 to 2.40. Same situation same error message. (On a Networksolutions host, not sure if this matters). Had to restore to 2.33. Searched on the Internet, found this post and another post answered in Germany...
Thanks in advance!

make sure the ASP_TAG is OFF. that was the cause of my problem

Also, some other config can cause this error :
/tmp folder not writable.
and safe_mod is on.

<?php

echo "TMP is_writable: " . is_writable("/tmp") . "<br />";
echo "SafeMode: " . ini_get('safe_mode') . "<br />";
echo "asp_tags: " . ini_get('asp_tags') . "<br />";

phpinfo();
?>

if that can help.

-Cristian

Worked! Thanks Cristian!
ASP_TAG was on. Since I'm on a shared hosting with Networksolutions. I did this to turn it off: create the php.ini file under cgi-bin folder with only one line: asp_tag=Off.
Unfortunately I had problems with my custom form after upgrading and had to downgrade AGAIN...

This is now fixed, thanks to pelse here: http://www.silverstripe.org/installing-silverstripe/show/286353

Thank you to all those who contributed.