First the happy news: If you use the nightlies or in the upcoming 1.7 release the backward of abs is implemented and works.
If you’re into the details of what happens: abs doesn’t support complex numbers in its backward in 1.6: it uses the formula dabs/dx = sign x , which is, of course, bogus for complex numbers. While trying the backward with this wrong formula it tries to take the sign of a complex number and that isn’t a thing, so you get the error you are seeing.which is the error you are seeing).
Best regards
Thomas