I have found the cause, it is because the field's name: Fixed_in_version instead of fixed_in_version (first capital letter is incorrect). After I have renamed the field it works fine.

For those who want do do it themselves, it takes just a simple SQL command to solve the problem:

ALTER TABLE mantis_bug_table CHANGE Fixed_in_version fixed_in_version varchar(64);

This is already fixed in Mantis 1.0.3. See original issue for details.