(I'm seeing this as well, but it had been working fine for quite some
time)

It looks like melpa is having issues:

https://github.com/melpa/melpa/issues/3833

Thanks!

I already opened an issue for this ( http://debbugs.gnu.org/cgi/bugreport.cgi?bug=23424 ). Hopefully we can put more info in that error message and make the source of that easier to trace...?