Redmine

My environment:

Environment:
  Redmine version                3.2.0.stable
  Ruby version                   2.0.0-p481 (2014-05-08) [x86_64-openbsd]
  Rails version                  4.2.5
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.8.9
  Filesystem
Redmine plugins:
  no plugin installed

Redmine production.log:

Could not copy subtask #1878 while copying #1874 to #1993 due to validation errors: JobNumber cannot be blank
Could not copy subtask #1879 while copying #1874 to #1993 due to validation errors: JobNumber cannot be blank
Could not copy subtask #1884 while copying #1874 to #1993 due to validation errors: JobNumber cannot be blank
Could not copy subtask #1886 while copying #1874 to #1993 due to validation errors: JobNumber cannot be blank
Could not copy subtask #1888 while copying #1874 to #1993 due to validation errors: JobNumber cannot be blank

JobNumber IS there in the subtasks. Both the source and target projects have the same fields.

Copying to the same project works, but copying to a different project has the above error.

Originally discussed here: Custom fields not copied over projects