I am not sure if I have the latest version of this add-on.
Download the latest version package
and compare your file's size with the size of the current addon_whosonline.php. If they differ, you may try replace addon_whosonline.php at least.
Yes, the issue with your file reminds me the recent problem I've fixed in this script time ago. Since then no such issues were reported, but I'm not sure if it really helped. So you may try and report here if you will not get this issue again with the newer version.