The Bug is in the file "filter_api.php".
You got the possibility to set customfields as multilist.
The seperator for this multiple values is a pipe(|).

In previous versions -I'm not quite sure in which one was the change- customfields were surrounded with pipes (e.g. |CustomValue|). In the last version singlevalues were saved without any pipes.

The filter function, however, is always looking for an entry with pipes.

SOLUTION (for Mantis 1.1.8, original condition):

1. open filter_api.php in an editor.
2. go to line 1210 (starts with: "array_push( $t_filter_array")
3. remove the pipes, Done!

Old Line:
array_push( $t_filter_array , db_helper_like( "$t_table_name.value", '%|' . db_prepare_string( $t_filter_member ) . '|%' ) );

New Line:
array_push( $t_filter_array , db_helper_like( "$t_table_name.value", '%' . db_prepare_string( $t_filter_member ) . '%' ) );

Thanks,

this solves the problem. But the Data in Database is with and without pipe... so the bug is at an other location...

This works in MantisBT 1.2.x, so closing.