I have an Excel Add-In saved to an .xla file in %appdata%\Microsoft\Addins. In the subject version of Excel it is checked on the Add-Ins list. One thing this addin does is create a drop-down menu in ThisWorkbook's Workbook_Open sub. But the menu is not created and there is no Add-Ins tab on the ribbon. No error messages.
This works fine in Excel 2010 and 2007 (at least), and worked fine on the 32-bit Excel 2013 that came preinstalled on this system before I nuked it. The really maddening thing is that if I get rid of this addin and open that same .xla file, either by double-clicking or using the Open command in Excel, the menu appears as expected. I get the same behavior if I instead open the source .xls file - it all works. I've also tried saving to a .xlam file instead of .xla, but get the same bad result.