Hello Geert
It doesn't make a lot of sense and I have no explanation to offer you but some things you might like to try:
If it is grayed then it may be a setting on the client computer, can you recheck the associations in file manager ?
Also copy an xls file to their computer and double click on it and see if it opens or has a problem.
Search your code for Disableaaf() in the faint hope that somehow you are turning off the export, although I think this makes it disappear not go gray
Delete your cpl directory to force a complete recompile and rebuild the exe again. This has helped us with a couple of strange problems running exe's at client sites.
Regards
Al