[toivo]

The location of the PHP error log is dependent on the type of server and you will need to talk to your hosting provider.

The line 1615 is in the file /includes/cron/emailintegration.php:

$plaintext_parser->set_parsing_language($touser['languageid']);

Workaround:

$plaintext_parser->set_parsing_language( 1 );

The idea of the workaround is to use the default language code 1. The original code depends on the contents of the $touser array which is for some reason incorrect.

Cheers,
toivo