This is definitely a bug in the back_maydeletehttp() 
function, file htsback.c, around line 449.

the line:
&& !check_sockerror(back[p].r.soc)

should be:
&& !back[p].r.is_file && !check_sockerror(back[p].r.soc)

(The value (invalid) -5000 should never be called in a 
network function (the handle has an invalid value))

I'll release a new version soon to fix this error - in the 
meantime, you can change the line to fix the problem.