Ok, here's what I do:
I enable attachments on a feature layer in ArcGIS Online.
I add content to ArcMAP through My Hosted Services.
I select Create Local Copy for Editing.
I add data from shapefiles to the local copy through Simple Data Loader.
When I try the last step I get the error "Cannot load into non simple data".
If I do not enable attachments on the feature layer (or disabling them after enabling them) in ArcGIS Online, the Simple Data Loader works just fine.
I cannot figure out what the problem is and I have googled like a madman, but can't find anyone else having this problem.
Just as a final note, I did do exactly as I described above about a year ago, and it worked properly, but now it doesn't. I am now using 10.3 but I'm not certain which version I was using a year ago. Maybe 10.1 if that makes any difference.
Can anyone help?