This may be related to viewtopic.php?f=6&t=1563 but in case it isn't I thought I'd start a new thread as I believe I have identified the cause and the steps to reproduce:

Can use the anime series 'Re:ZERO -Starting Life in Another World' to reproduce
In Sonarr:
Manually search for Episode 1
Choose a recent Director's Cut result
This results in the additional dialogue below in an overlay popup window in Sonarr, at which point you can choose Grab and the episode will be downloaded successfully
Grab Release
Sonarr was unable to determine which series and episode this release was for. Sonarr may be unable to automatically import this release. Do you want to grab "EPISODENAME"?
Performing the same manual search and trying to download the same episode via nzb360 results in a giant full screen error message that begins
'Couldn't download release. Try again. Error: {"message": Value can not be null.'

Let me know if you need any additional information. Thanks.
I've been having the same issue for a couple months. It started after the pop up dialog about a rejection reason with the 'download anyways' button started popping up. It says it downloads the release, but never shows up in history or moves to sabnzbd. Sometimes it throws the error posted earlier.

I have to manually add because I have fairly strict filters on which scenes I download from. If the episode isn't uploaded by any of my preferred scenes, I have to manually download. Not being able to do that anymore is incredibly annoying.
Yeah I'm hoping this is on Kev's radar and is something that can be addressed.

The bulk of my shows download automatically. The one's that do not are typically because of strict scene mapping restrictions too, and for most I cannot manually start the download from nzb360 due to this error, so I have to load Sonarr to do so. Definitely not as convenient as being able to do it straight from the app. So hoping a fix is in the works.