This issue has happened a lot over the last few months with certain Revit models. I tried different computers to check if the PC was the issue. Something within the Revit models themselves seem to be the issue. There is plenty of hard drive space on all the computers I've tried. If I open a different model on the same computer, the Revit-Connect Add-In will upload with no issues. I suspect there is some sort of element or object in the Revit model that the Connect-Revit Add-in doesn’t accept. A few weeks ago, I encountered this with a model with contour lines and received the error message. I deleted the contour lines from the Revit model and the model uploaded successfully. Today I received the error message with another Revit model, and I deleted some generic geometry bounding boxes and the model uploaded successfully. Unfortunately, it's not always obvious what I should remove from the Revit model to make the model publish successfully. I would think others are experiencing this issue as several people in my office have encountered it from time to time.