Hi,
Are you building with runtime packages?
If you are, can it be that it is finding and older bpl which doesn't include XlsxAdapter?
Can you try to build without runtime packages to see if it fixes it?
I don't have many other ideas of what can be going wrong.
I've found a similar issue here:
Where it looks like it was fixed by changing the order of uses/includes. I wonder if changing the order makes some difference in your app?
I also wonder what happens if you register the class manually?
initialization
RegisterClass(TXlsxAdapter);
But I am really no expert in intraweb, maybe someone with more experience on it can help further.