Please help, it's getting worse. See attached screenshot and you'll see that all issue details apart from custom fields are missing in issue details view. When editing an issue, all is available, but not in detail view.

What could cause that?

The $g_bug_view_page_fields configuration option (and other similar _fields options) changed since 1.2.0RC2. Where you once needed to use constants in the array, you now have to use strings. For example, to show all fields on the view page:

$g_bug_view_page_fields = array (
    'id',
    'project',
    'category_id',
    'view_state',
    'date_submitted',
    'last_updated',
    'reporter',
    'handler',
    'priority',
    'severity',
    'reproducibility',
    'status',
    'resolution',
    'projection',
    'eta',
    'platform',
    'os',
    'os_version',
    'product_version',
    'product_build',
    'target_version',
    'fixed_in_version',
    'summary',
    'description',
    'additional_info',
    'steps_to_reproduce',
    'tags',
    'attachments',
    'due_date',
);

Jurgen reported on IRC that updating $g_bug_view_page_fields to contain strings instead of constants (see config_defaults_inc.php for an example of how $g_bug_view_page_fields should be defined) solved the problem.

Resolving as "no change required".