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.