More info. Here is the xml code being parsed:

<mso:customUI xmlns:doc="http://schemas.microsoft.com/office/...urrentDocument" xmlns:mso="http://schemas.microsoft.com/office/...ls><mso:button idQ="doc:test1_1" visible="true" label="test1" onAction="test1" imageMso="ListMacros"/><mso:button idQ="doc:test2_1" visible="true" label="test2" onAction="test2" imageMso="ListMacros"/><mso:button idQ="doc:C:_Users_ed_Desktop_test.xlsm_test3_1" visible="true" label="test3" onAction="C:\Users\ed\Desktop\test.xlsm!test3" imageMso="ListMacros"/></mso:documentControls></mso:qat></mso:ribbon></mso:customUI>

If I edit out the highlighted in red "C:_Users_ed_Desktop_test.xlsm_" and the "C:\Users\ed\Desktop\test.xlsm!" for macro test3 as I have already done for macros test1 and test2, the workbook opens without error and the QAT buttons are there and work.

For some reason, the "C:\..." location imformation is being generated and not being parsed correctly.