Guys, this seems to be not a GIMP Portable problem, but the GIMP itself.
I'm experiencing this issue on both local and portable versions of GIMP 2.8
I've copied the help for 2.6 (the latest available for download now)
to the folder App\gimp\share\gimp\2.0\help - so my actions seem to be right.
When I go to the Preferences - Help System - General - User manual, it says "There's a local installation of the user manual.". But it doesn't work.
Please tell me - can you reproduce this error?
Let's hope that the updated help file for 2.8 will fix this...
Another little issue that I've found:
after each launch GIMP Portable leaves a file with a name like:
and does not delete it.