I had the same problem, and I fixed it by making a .cmd file with the line below, and changing the app that opens the xlsx files to that cmd file.
start "" "C:\Program Files\Microsoft Office 15\root\client\AppVLP.exe" "C:\Program Files\Microsoft Office 15\Root\Office15\EXCEL.EXE" %1
I saved the file with a name like "Open_Excel.cmd" in my "C:\Users\myusername" directory.
To have the excel files I double click call this cmd file I right clicked an .xlsx file and selected Open With > Choose another app.
In the pop up I selected "Always use this app to open .xlsx files". Then I clicked "More Apps" and scrolled to the bottom of the list and selected "Look for another app on this PC".
In the window that pops up I navigated to where I saved the .cmd file, selected the file, and clicked "Open".
I came up with this solution because I noticed that when I right clicked the "Excel 2013" icon in the start menu and clicked "open file location" it took me to an "Excel 2013" shortcut. When I right clicked the "Excel 2013" shortcut and went to "properties" I noticed that the "target" was actually two executable files. The two files the ones we call in the cmd file we made (AppVLP.exe and EXCEL.EXE).
The file calls "start" which executes "AppVLP.exe", and passes "Excel.exe" as an argument. The "%1" lets the .xlsx file that's double clicked be passed to the "Excel.exe" as an argument (otherwise you get the grey excel window). Lastly, the empty quotes ("") lets the whole thing be called (otherwise you get the pop up "your operating system is not configured to run this program")
Hope this is helpful!