Thread:Extension talk:WYSIWYG/Category button not working correctly/reply

In my wiki it shows "unknown object" too when hovering mouse over the C -icon, I was unable to find reason for this.

When category name has spaces and you save the page category will be created so that spaces in category name are converted to underscores. This does not take place in category dialog, it takes place somewhere else. In category element (icon) name of category seems to be stored with spaces. I guess this is because it is more readable that way.

I made modification so that spaces in category name of category element are converted to underscores, it seems to work in my wiki. Perhaps you could try if it works also in your wiki (#6).