can you post the model??
Some time ago when i start my journey with wings i have such error and i remember i manage to resolve the issue, but since it was a long time ago I don't remember what i do ;/.
Maybe when i see model i will tell you what is wrong.
edit:
I found some old post, and this error could be because of several vertices sharing the same location, try to use 'Clean up' option when you have all model selected. But best will be if you post the model here.
