I am having the same problem with the F1 Help key.
I upgraded from v18.15870 to the following version
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.15991 (Git)
Build type: Release
Python version: 2.7.14
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.2.0
Locale: English/UnitedStates (en_US)
and I get the same error message when pressing the "F1" key.
It was working correctly before the upgrade.
Now I also get the same error when I use v17 on the same PC.
Anyone have any ideas on solving this.
The help documentation files are in their correct location in the "DOC" folder. If they are not then a different error message is displayed indicating so.
I tried going back to the previous build of version 18 and this did not solve the problem and now version 17 is doing the same thing.