E810: Cannot read or write temp files
E97: Cannot create diffs

FIX (that worked for me):
In MyDiff() of _vimrc, go to line 13 and change the two single quotes to double
quotes. This allows the pattern in the IF-test to match.

So change:
if &sh =~ '\<cmd' (fails)

if &sh =~ "\<cmd" (works)

Or keep single quotes but escape the backslash:
if &sh =~ '\\<cmd' (also works)

from https://groups.google.com/forum/#!msg/vim_use/SR12FCxkQYg/aZIGO33waN0J