The error message I got (not in the workbench windows, but in word) was "Protected Tag!" I still think it's a mystery, since the other two files in the same project are ok.
Trados tags in Word are protected to prevent unintentionally damaging them. If you try to edit a tag, that's what you get.
About importing my tm for this project from Wordfast: Workbench gives me the error message "5094 TUs read in 3 seconds: 0 updated, 0 added, 5094 invalid".
Sounds like a language/locale inconsistency issue - check whether the language codes match exactly. What languages/locales does your TM involve? Assuming it's English to Swedish (based on your working language pair), could there be a discrepancy between US English (EN-US) and UK English (EN-GB)?