Closed Unassigned: Workbook.SaveAs error [9106]

Hi All,
I am trying to use ClosedXLM in a Dynamics NAV environment to speed up some Excel workbook/sheet production. It is a bit complex situation with a template, NamedRanges, and also macro's to fit pictures in cell's.
Everything looks working perfect, speed is 4 times the old solution with an Office automation, till the workbook is saved to a memorystream, when I get the following error :

Microsoft Dynamics NAV
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This message is for C/AL programmers: The call to member SaveAs failed: '.', hexadecimal value 0x00, is an invalid character..
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OK
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Changing saving to file produces the same error. The result file is written to file, but the sheet.xml is invaled.
Apparently the saving process stopped near the end op the process. Validation the XML produced the an error on missing closing tag's as : <x:oddHeader> (and others)

I guess the template is not correct or something like that, but the strange fact is : the automation solution is still working fine.

Any suggestions?

Benno
Comments: Closed per user's comment that they were able to fix the issue on their own.