A bug on the Angle to horizontal tool has been reported by samfuen (thanks !).
The bug affects versions 0.8.21 and 0.8.22, possibly older versions.
A corrected version of the tool is available here : 8 - Angle to horizontal.xml
Fixing your Kinovea is simple, download the file and replace the old one in the installation directory, sub directory DrawingTools.
For instance: C:\Program Files\Kinovea\DrawingTools\ or C:\Program Files (x86)\Kinovea\DrawingTools\ for 64bit systems.