Ok, I think I found the problem yesterday.
I moved the site to another host and the problem was still there.
Then i decided to restart with a fresh Joomla/Hikashop install.
After a few tests everything worked fine, untill menu items (categoryview & product view) started to
return pages without template. Tested default templates, the problem was not solved.
Then I thought, maybe my downloaded files may be corrupt..
So I overwrote all Joomla files, and reinstalled Hikashop (essential)
but the problem was still there, so then I downloded the new version of Hikashop essential and installed it
in Joomla on the server, and from that point everything worked ok.
So maybe the downloaded version of Hikashop I had used just had a little defect I think.
That is my explanation, because switching from Essential to Business and/or back for testing did not make any positive changes.
It had to be removed, and installed again with the latest version.
Anyway, everything works fine now!
Best regards, Roy