Probably you use old version of HostMonitor? Import function worked correctly with master/dependants tests only in case master tests specified before dependants. I think it was corrected in version 3.40
As I see HostMonitor imports "exportfile.txt" without any warnings. When you import "importfile.txt" HostMonitor displays error because specified Master test is not defined in the import file. All specified tests must be defined in the same import file.