Incorrect text display in Help Viewer

I am running a 2012's MBP, updated over the years continuously to the latest version of macOS (currently 10.13.3).

Even though all of the major macOS updates ran amazingly well, with the latest update to High Sierra, the help viewer's content is displayed incorrectly: The complete text is shown in one line of the left hand side of the window, having a line break after each letter, making reading it almost impossible of course.

What I tried so far is deleting help viewer's cache (as discussed in [1]).

Interestingly this problem only applies to my existing "old" account on the machine. Adding a new user and using the help viewer with that user being logged in works as expected. So that problem is not a system-wide problem, but is rather caused by some user-specific preferences. Due to a lot of customized settings, I would appreciate to keep using my existing account. Any help is highly welcome.

[1] https://discussions.apple.com/thread/6524067