I am getting occasional system crashes when my customer is oking an invoice. Version 8.1 2016-03-31

It happens to different people. Can go days without any issues. So very hard to track the cause.
Here is a snippet from the Hansa log. Same message all the time just the user initials differ.

Please can anyone advise what this may be?

2017-07-20 10:32:21 JJS CompareMatrix (vc name on next line) sizes differs arsz: 3680, h1: 5312, h2: 5312
2017-07-20 10:32:21 JJS CompareField vc: IVVc field: Math differs, "" <> ""
2017-07-20 10:32:21 JJS CompareMatrix (vc name on next line) sizes differs arsz: 3680, h1: 5312, h2: 5312
2017-07-20 10:32:21 JJS EInvoiceingEnabled err1 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err3 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err1 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err3 res=1

2017-07-20 10:32:21 JJS EInvoiceingEnabled err1 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err3 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err1 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err3 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err1 res=1
2017-07-20 10:32:21 JJS EInvoiceingEnabled err3 res=1
2017-07-20 10:32:21 JJS The data stack is full
2017-07-20 10:32:21 JJS dump of call stack
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/RateTools.hal: DIVM4ONERATE
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/RateTools.hal: MULBASERATE
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/RateTools.hal: MULRATETOBASE2
2017-07-20 10:32:21 JJS call stack entry: hal/Reports/DayBalTool.hal: ADDEUROCURNCYNOMARTRROW
2017-07-20 10:32:21 JJS call stack entry: hal/Reports/DayBalTool.hal: ADDEUROCURNCYNOMTRROW
2017-07-20 10:32:21 JJS call stack entry: hal/Reports/DayBalTool.hal: ADDEUROCURNCYTRROW
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/IVTransTool.hal: ADDIVTRANSROWSNEW
2017-07-20 10:32:21 JJS call stack entry: hal/RActions/MakeTransFromIV.hal: MAKETRANSFROMIV
2017-07-20 10:32:21 JJS call stack entry: hal/RActions/IVVcRecAction.hal: IVVCRECORDCHECK
2017-07-20 10:32:21 JJS runtime error, hal stack is:Stack full
2017-07-20 10:32:21 JJS dump of call stack
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/RateTools.hal: DIVM4ONERATE
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/RateTools.hal: MULBASERATE
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/RateTools.hal: MULRATETOBASE2

2017-07-20 10:32:21 JJS call stack entry: hal/Reports/DayBalTool.hal: ADDEUROCURNCYNOMARTRROW

2017-07-20 10:32:21 JJS call stack entry: hal/Reports/DayBalTool.hal: ADDEUROCURNCYNOMTRROW
2017-07-20 10:32:21 JJS call stack entry: hal/Reports/DayBalTool.hal: ADDEUROCURNCYTRROW
2017-07-20 10:32:21 JJS call stack entry: hal/Tools/IVTransTool.hal: ADDIVTRANSROWSNEW
2017-07-20 10:32:21 JJS call stack entry: hal/RActions/MakeTransFromIV.hal: MAKETRANSFROMIV
2017-07-20 10:32:21 JJS call stack entry: hal/RActions/IVVcRecAction.hal: IVVCRECORDCHECK
2017-07-20 10:32:21 JJS HAL call stack dumped, check log file for details.
2017-07-20 10:32:21 JJS Stack full