iPad error 52

I have an iPad, can't remember what version it is, but it is the first one with a retina display. When (trying to) upgrade to iOS 9.3.5 via iTunes, it seems to hang, then I get an error 52 message. This keeps happening when I try to restore from my computer. I've tried using other USB cables, using other USB ports and using another computer, all to no avail. Any tips>