I had the same issue ....

I had the same issue.... I tried the whole remove updates thing, I tried the delete apps, turn off gmail, gtalk etc. etc. etc. What you need to try before all of that is to confirm that Media background data is not turned off. Download Manager is a component of Media. Settings>Usage>Media make sure Media is not ticked. I ticked this when I was tinkering with the settings... and for 3 weeks I couldn't work out why I couldn't download apps via 3g/4g.