what is your operating system?
What is your exact version of InDesign CC ? (Look under menu: "About InDesign")
Are there special characters in the image file names?
Some umlauts perhaps?
Try creating a tab delimeted text file to get the merge going.
Saved with a text editor to Unicode (UTF-16).
I run OS 10.11.2 and InDesign CC version 10.2.0.69 no Umlaute. I will try what you suggested and get back to you.
I still get the same error message "The data source references one or more missing images......" I wish I could attach my spreadsheet here, but I don't see a way how to do that.
However, when I copy and past (only pasting values) of just the image column into a brand new excel spreadsheet and then copy it back into the spreadsheet, I get at least 80% of the picture to merge, still not all of them but most. So it seems that it HAS to be something in the excel file that prevents merging, even if I remove all formatting, it won't recognize the pics until I remove the column, paste special, and then paste back. Weird right?
If you want to share the spreadsheet, upload it to Dropbox (or a similar service) and post the link.
Or contact me through Personal Message and provide the link there. Or we could exchange mail addresses…