I think that Monarch 11 was released prior to Office 2013, although only just, and some of the finer points of Office 2013 formats aren't included in that release. This may impact both opening Excel files as data sources and writing out to them with features like named ranges, print headers, and so on.
I know that this is being worked on, and that v12.4.2 which should be released "this week" will address some of them, but I suspect you will need to upgrade to solve the problem.
Thank you for the feedback. I was thinking I may just need to upgrade. I am running Monacrh Pro 32Bit 11.8 and Windows 7 32Bit with Microsoft Office 2013. When the conversion to 2013 Office was forced last week, my previous version of Monarch 11.2 would not work with any new files created in Excel 2013 (.xls or .xlsx) when opened as a database, but would open files created in Excel 2010 (both .xls and .xlsx). I contacted local IT and another user familiar with Monacrh. They had a patch for 11.8. They Installed it and 2013 .xlsx files wil still will not open, but 2010 .xlsx files will. When I attempt to open 2013 .xlsx file it acts as if it is thinking about loading and then tells me that Monacrh is not responding. I have to force it to close. I did figure out that I can open any version of .xls. I have repaired MIcrosoft office, and uninstalled and reinstalled Excel 2013 twice. I dont know how to proceed, and no one here has enough knowledge with Monarch to assist me. Open to all suggestions.