I had the same problem when I re-opened an old Afx Project.
Afx linked to a non self contained reference file from FCP.
In the meanwhile I had consolidated the FCP project (media manager) and deleted the old footage from the disk.
The problem was that the reference file linked to footage that didn’t exist anymore.
I exported a new reference file from the consolidated FCP project and replaced the old reference file.
Then I reopened Afx, problem solved.
So for short:
I replaced a reference file that linked to non-existing footage.
I hope this is any help to your problem.
Karel