Interesting. I'm not sure why so many malformed calls are happening, I've added a PR to give us more information: https://github.com/NebulousLabs/Sia/pull/976/files
As for the <nil> errors, @allejuppa already created a PR (and it was merged) to give the correct error information.
I can't be of too much help, except to suggest that most of the network is experiencing a similar number of errors in their logging. I think at this point we've addressed some of these errors, but we're continuing to work on the renter and host protocol right now.