Throwing "ElementNotSetException " intermittently while running Workflow

Dear All,

We have designed overall main workflow along with multiples manageable sized (size less than 4 mb) workflows being invoked using Invoke Workflow .

While running overall main workflow, it is intermittently giving exception / errors as “ElementNotSetException” - PFA screenshot for detailed exception.

Also it is giving out of scope error, it is like we should indicate the activity again and then it works fine.

Please help and suggest how to get rid of this exception and such issue should not reoccur .

Appreciate early help.