Binding from workflow to work item aborted , when approver got changed

Hi Experts,

I have a workflow which is running since years  and there were no issues with it.

During the approval process at Director level it is throwing an error for a particular user( not for other Director role users).

Recently I could see an error for 2 documents and it's description has been pasted below.

Confirmation of work item 000011957046 flagged for later execution

<span data-mce-style="color: #000000;" style="color: rgb(0, 0, 0);">Error when determining attribute 'NAME' of object instance '[BO.USR01.BOERHQA]'</span>

<span data-mce-style="color: #000000;" style="color: rgb(0, 0, 0);">Error in the evaluation of expression '&amp;LastWorkitem.ExecutedByUser&lt;???&gt;.Name&amp;' for item '28'</span>

<span data-mce-style="color: #000000;" style="color: rgb(0, 0, 0);">Source (expression '&amp;LastWorkitem.ExecutedByUser.Name&amp;') of binding assignment is not available</span>

<span data-mce-style="color: #000000;" style="color: rgb(0, 0, 0);">Step 210 of WS90000041: Binding from workflow to work item aborted</span>

Workflow 'Subworkflow for N-Level Approval' step number 11957048: work item could not be created.

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Kindly help me out with your valuable answers to address this error.

Thank you,

Regards,

Giri