Most likely this is RAM problem.
If you open Explorer with Details option ON (to be able to see file size) - you can see it. Also Task Manager > Performance window shows this well.
Just export your Su model to any bitmap format - let's say jpeg - and watch Explorer and Task Manager windows. You'll see (for a few seconds) temporary jpeg file wich size will be about 10 times larger than final one (i.e. it will go up to 300 MB if final one will be about 1-3 MB!!!). Also RAM consumption will jump by 50% or more.
I think this is caused by the way SU processes vector data to raster formats.
As possible solution I export in two or three steps.
First one - with edges OFF, texture and shadows ON, AA also ON. Resolution - maximum supported or the one I need.
Second export one - Edges ON, the rest is OFF, AA also OFF. Resolution is usually twice of First export.
Optional third export - I render only shadows (no textures, no edges and First export without shadows). Resolution is same as First export.
The rest is done in Photoshop - resizing, layers composing etc.
By using this method I achieve two goals - higher resolution output from SU and more control over texture/edges/shadows interaction.
PS. AA - anti-aliasing. Photoshop is much better in this then SU.
And the Save Failed feature "works" for me since SU 4
(teapot_ae)