H Damien, at first read this did not sound familiar. This is certainly not intended behavior :-) 

I do remember though from a few years ago when I worked via satellite connection (also a bit slower connection with a long latency) that such messages frequently appeared. Not sure if it was exactly the same but at least my ‘Publish’ actions failed I remember. My remedy was this: When the publish workflow window appears do not instantly click the ‘Publish’ button. Wait until the lookup fields have fully populated and give the window a few more seconds to fully load. Then click 'Publish’. This improved my chances on a successful publish action and reduced error popups I remember.

Let me know if this also works for you. If so, we might have to suggest to Nintex to not enable the Publish button until the form has fully loaded. 

@DamienT did it work for you? If so, appreciate if you mark this as a ‘solution’ after which I can add this to the uservoice as a change request.

Hi WouterT, thanks for the suggestion; I didn’t have any success when waiting for a period while on the publish screen before pressing publish (waited 30 seconds). 

It’s a super weird issue that’s for sure.

It certainly is. Sorry that I cannot help you any further with this.

This may seem like an odd thing to try but I’ve had some funny save errors in the past as well.

Try a browser clear cache and reload the workflow cloud site.

Another alternative is try a different browser or create a new browser profile.

That should hopefully rule out any corrupt cache or cookie issue.