The links in the help system seem to be a bit confused. Often it will get in a state where ''it can not get there from here''. I'll try to express the behavior as best as I can.
I browse the help. I look at ''Mathematica Book'':''Reference Guide'':''Basic Objects'':''Expressions'' And read the topic. Then I move to ''Mathematica Book'':''Reference Guide'':''Input Syntax'':''Expressions'' using the combo box panes in the help browser. No text will appear. If I close the help system, and browse immediately to the second topic, it does appear. However, if I browse to the first topic, it won't appear. (It appears to me that the topics are divided into two books, and can't find each other.)
I am using Windows NT 4.0, and Mathematica 18.104.22.168x (Student).