Is the update icon wheel still spinning? This happened to me recently and was some dodgy artwork.
I found the file by looking one-by-one at the tracks I'd just added, but I only had about 10 new files. What might help though is opening the card contents in your file explorer (Windows / Mac) and do a wildcard search (*.* in windows). If you then sort the results by file path you will see roughly the order the X5 scans in I think. Count down to around 1500 tracks (not including folders) and look for new tracks in that area (1500-1600) that might be the culprit. Check tags of new files one-by-one to find irregular tags or artwork.
Hope this makes sense.