Comment 1 for bug 234240

This issue probably only happens when you use the mouse (or touchpad/trackpad) to click on a location where you want to type text (but not if you use the tablet pen to select the location where you want to type text), right?

If so: upgrade to xournal version 0.4.2.1 to fix the issue (which is actually a bug in the X server causing invalid events to be generated, but the newer xournal includes a workaround), or upgrade the X server (if the issue has been fixed in newer versions?), or, temporarily, uncheck the "Enable XInput" button in the Options menu (the price to pay being a loss of quality when you use the stylus to write on the screen).

Denis