I've been able to import in the past an invoices iif file into Reckon. Of late, the file seems to crash Reckon Hosted when i import the iif file. I'm exporting the iif file from Briefcase and importing it into Reckon Hosted.
Importing via IIF is never a good way to do and when something goes wrong, it is really hard to identify the issue. I can take a look at the file you tried to import and see if I can pinpoint the issue.