I had the same thing for one of my LORs. I called AADSAS and the rep told me that an Incomplete status can be due to one of the following:
1.) Professor did not sign the letter
2.) Professor did not sign the envelope flap that carried the letter
3.) The letter got damaged in the mail
The rep told me that they often look past 2 and 3 unless it's a glaring issue. I suppose the only thing to do would be to contact your prof, give them another AADSAS LOR form, and have them send the letter again with 1 and 2 in mind.
Hope that helps!