If I tried to run any tasks it would just go into a endless spinning icon.
I have now got another spare Pi to run some tests on. Rebuild fresh with Sonarr/nzbget and now it seems to be behaving itself! Will monitor over the next few days and if it starts with the issues again will post the info.
Hey, I took a look and there is definitely something weird in the DB, but that should be a quick fix to resolve. The easiest way to fix it is to stop Sonarr and then open up the DB in a sqlite editor and run UPDATE Commands SET Status = 1 WHERE Status = 0 (This will terminate the pending tasks), then start up Sonarr.
Which rPi are you running Sonarr on? It should run on them all, but performance would certainly vary.