Classic mistake, though mistake #1 is not ... using excel.... basic mistake is to leave the data as is spread on lines.
1400 cases spread over 65000 rows roughly 50 rows per case, if the data was managed properly changing the 50 lines to 50 columns it would have allowed 65000 rows and the problem would hopefully never occured.
I dont see a problem reporting this data with excel, xls or xlsx.... worse yet the file most likely is at the core a text file with the xls extension.
As a result this problem, but someone somewhere has been ignoring the "file not loaded completely" error.
It is not the first time this cuases a problem, excel should hard fail to load a file that is too big.