Such problem may appear in KSWEB 3.63.
It is related with the fact that path to log files includes not your true path to sdcard on your device.
Go to KSWEB settings and click to the first setting “Path to external storage”. Select one item from the storage paths list. Restart KSWEB.
Also you can fix the problem in other way manually:
For example for lighttpd: edit lighttpd configuration (Lighttpd tab -> Configuration Edit):
server.errorlog = “/path_to_your_sdcard/ksweb/log/lighttpd/lighttpd.log”.