You may have to check the ULS logs while testing to see if any errors are being spit out behind the scenes.   This should give you a better indication of what might be taking place.

Thanks,

Mike

Thanks Mike.

I found the following error:

05/17/2016 14:31:12.48 w3wp.exe (0x19FC) 0x1568 SharePoint Foundation General c3pa High Batchmgr Method error. Errorcode: 0x41ff9f40. Error message: The user does not exist or is not unique. 3e2ba1b1-c305-4e7b-9e2c-5ab6b63cf067

There is indeed a mailbox involved that is not unique. We have to resolve this.

But why is it working intermittently? This mailbox is being used all the time and it fails very rarely!?

While this error is logged in ULS the web service action does not throw an error.

Is there a way to configure the web service call to catch this particular error?

Thank you.

Hi Thomas,

I've seen similar things before, and sometimes just selecting the"right" user is enough to fix it.  It can get pretty confusing when two or more objects have the same name.   I  can't be 100% but it might be that different web front ends are handling the workflow (do you have more than one?) and thus you see the different behavior.   I've seen similar things along those lines.   If you  are  already setting the webservice to  catch the error then you probably  won't be able to log it further as far as the Nintex designer is concerned.   You could turn on verbose logging to get more output in your ULS logs which may or  may not help.  Hopefully this is enough to  get you started down the right path.

Thanks

Mike

Thank you Mike.

I know what you mean. I also experienced some strange things and scratched my head about SharePoint and workflows.

I marked your answer as correct.