I think I also have the same problem. Trying to upgrade from version 1.1.1, a custom field called "In ReleaseNotes" has stopped working.
It's an enumeration with possible values:
"|Yes|Not required"
and is only required on Resolve, not on Report or Update. With Mantis 1.2.1 it now produces the following error:

"APPLICATION ERROR #11

A necessary field "In ReleaseNotes" was empty. Please recheck your inputs"
Thanks,
Andy

Or make it non mandatory or show on the create report

@cas, yes obviously that would work but it's not really a suitable solution. None of the both suggestions.

It doesn't seem to be all types that are affected. Looking at the code in bug_report.php there seems to be a block which stops enumerations, lists, multilists and radio buttons!

Line 106:

Produce an error if the field is required but wasn't posted

    if ( !gpc_isset_custom_field( $t_id, $t_def['type'] ) &&
        ( $t_def['require_report'] ||
            $t_def['type'] == CUSTOM_FIELD_TYPE_ENUM ||
            $t_def['type'] == CUSTOM_FIELD_TYPE_LIST ||
            $t_def['type'] == CUSTOM_FIELD_TYPE_MULTILIST ||
            $t_def['type'] == CUSTOM_FIELD_TYPE_RADIO ) ) {
        error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
        trigger_error( ERROR_EMPTY_FIELD, ERROR );
    }

Really not sure what this meant to be doing and it's certainly very different from the version in 1.1.1 which doesn't have any of these explicit checks.

Andy

As far as I can see removing these tests:
$t_def['type'] == CUSTOM_FIELD_TYPE_ENUM ||
$t_def['type'] == CUSTOM_FIELD_TYPE_LIST ||
$t_def['type'] == CUSTOM_FIELD_TYPE_MULTILIST ||
$t_def['type'] == CUSTOM_FIELD_TYPE_RADIO )
makes it work as I want it to!