Do you have all of the other required fields completed? There are several in the email sections. If so, try removing the participant and re adding.

Thanks

I add the object, add a recipient and a task title, save, open again and can't save again. I had a look through and all required fields appear to be entered. I also tried removing the recipient field and re-adding with no luck.

It's only the two task actions that won't let me save when editing! Very strange.

I've logged this one with Nintex so will post back the resolution soon.

How customized are those actions? Could you delete one of them and add the action back and reconfigure?

Not custom, just the out of the box "Assign a task" and "Start a task process" actions. I'm pretty sure it's related to my custom JavaScript on my masterpage, but not getting any errors that help to resolve the issue. I also tried re adding the Nintex app which didn't help.

I tried the actions on the same tenant in a different site collection which was a blank publishing site and it worked, so def something to do with my customised site.

It can be possible that it would interfere. Could you create a subsite that does not inherit the masterpage and test a workflow there?

So, to summarise the fix and the problem:

- A wiki site template was being used since Microsoft don't support publishing sites at the root web in the site collection

- I programmed the feature activation to match that of a publishing site, but missed one under site features - Workflow Task Content Type (Adds the SharePoint 2013 Workflow Task content type to the site.)

So in summary, ensure the workflow task is activated if your not using a site template which has it activated by default!

Thanks everyone for your help, I hope this helps someone else down the track.