Hi,
probably your hosting forbids setting memory limit or time limit from php script. It can be also caused by very big file, or your host has too little memory... 500 error is usualy because php script was killed.
Some hints:
XLS import is memory consuming, try using CSV (excel can convert xls to csv).
You can also try with 1 row csv file.
If 1 row csv doesn't work try comment out lines 192-193 and 374-375 from modules/Premium/Import/Import_0.php...
Paul