What I gather from the site is someone is trying to access the addon_whosonline_data.php file directly while its being updated by another user and causes the syntax error.
This process actually happens instantly on your forums page ;-) That's why in the add-on's core, there is implemented a "rename" command, which renames a copy of that data file before updating it. It's the fastest function operational system can handle.
If there are problems like you described, it only could mean at some point your server has a break and didn't get the renaming/updating the file properly. Why it happened? Who knows...
How much users instantly online do you have on your forums btw?