Hi,
The source of the problem is bad geometry. Likely you imported the or parts of the model from another CAD system, but not necessarily.
When running with an existing mesh the mesh is compared to the geometry to see if it is a complete mesh. If there are significant approximations of the mesh to fit the geometry then this check will fail and it will try to re-mesh, however due the underlying existing mesh it has a harder time. The mesh diagnostic is the primary way to find where the geometry needs fixing so "A complete mesh already exists..." is successful. Not every error needs to be fixed so it takes some judgement and trial an error to find the root cause geometry problem. Also, if I remember right there are some mesh settings that may not be treated exactly the same pre-meshing vs mesh at analysis run, so sometimes one or the other way is more successful at meshing for that reason. The diagnostics are pretty good at highlighting the geometry with trouble but in a pinch you can do the old method of a binary search by cutting your model in half and seeing which half works and which half doesn't and then continuing to divide in half until you can identify what geometry is the culprit. We use import data doctor a bunch because our models are generally imported and almost always have trouble areas to fix. If you want more specific help and you are allowed to upload the model, someone at the community could take a closer look and show the exact issue and get you on your way again and better for the experience.
