I'm using 1.1.1

I've got the same problem with the so called "stable version" 1.1.6.

a similar report is this:
http://www.mantisbt.org/bugs/view.php?id=6661
where someone announced that there is a new filter_api that solves the problem. however that filter api belongs to another version and does not work with 1.1.6

in this thread a solution is suggested:
http://forums.mantisbt.org/viewtopic.php?f=3&t=2137&start=0&st=0&sk=t&sd=a

because that didn't work as well for me, i developed a new working solution:

just open
/mantis/core/filter_api.php

and search for the line:

if sorting by a custom field ## was error, now changed

if ( strpos( $csort, 'custom' ) === 0 ) {
$t_custom_field = substr( $csort, strlen( 'custom' ) );
$t_custom_field_id = custom_field_get_id_from_name( $t_custom_field );

and then replace this:
$t_join .= " LEFT JOIN $t_custom_field_string_table ON ( ( $t_custom_field_string_table.bug_id = $t_bug_table.id ) AND ( $t_custom_field_string_table.field_id = $t_custom_field_id ) )";

by THIS:
#==== WORKING: =======
$t_join = " LEFT JOIN $t_custom_field_string_table ON ( ( $t_custom_field_string_table.bug_id = $t_bug_table.id ) AND ( $t_custom_field_string_table.field_id = $t_custom_field_id ) )";

Have fun,

meejiuus

This problem is still present in 1.1.7.

still present in 1.2.0a3

Workaround:
dont use - or other Signs in the Name of the fields.
Example:
noGo -> ok
no-Go -> not ok

_ Works!

Example:
no_Go -> ok

See issue 5859 for a better solution.
Solution from meejiuus will not consider the sorting of all custom fields columns.

This can be solved with alias as tablenames:

In core\filter_api.php

Old Code:

if sorting by a custom field

            if ( strpos( $c_sort, 'custom_' ) === 0 ) {
                $t_custom_field = substr( $c_sort, strlen( 'custom_' ) );
                $t_custom_field_id = custom_field_get_id_from_name( $t_custom_field );
                $t_join .= " LEFT JOIN $t_custom_field_string_table ON ( ( $t_custom_field_string_table.bug_id = $t_bug_table.id ) AND ( $t_custom_field_string_table.field_id = $t_custom_field_id ) )";
                $c_sort = "$t_custom_field_string_table.value";
                $t_select_clauses[] = "$t_custom_field_string_table.value";
                }

New Code:

if sorting by a custom field

            if ( strpos( $c_sort, 'custom_' ) === 0 ) {
                $t_custom_field = substr( $c_sort, strlen( 'custom_' ) );
                $t_custom_field_id = custom_field_get_id_from_name( $t_custom_field );
                $t_join .= " LEFT JOIN $t_custom_field_string_table $t_custom_field_string_table$i ON ( ( $t_custom_field_string_table$i.bug_id = $t_bug_table.id ) AND ( $t_custom_field_string_table$i.field_id = $t_custom_field_id ) )";
                $c_sort = "$t_custom_field_string_table$i.value";
                $t_select_clauses[] = "$t_custom_field_string_table$i.value";
                }

Perhaps could you implement this in the next release.

Regards