In what way did the upgrade go wrong? Could the install have gone wrong again? e.g. If something prevented the help file being written to disk then that might explain things. Reinstalling 10.5 again might be worth a try.
It's also possible that something has broken the Windows HTML-help viewer's installation. The Help file does not display correctly FAQ has some possible solutions.
(I don't know if it's just coincidence but I'm starting to see a correlation between people who use Revo Uninstaller and people who have weird installation problems, FWIW. Could be explained by those people also tending to install & uninstall lots of random stuff, of course.)
I then did re-install but got again an error message while rebooting. Hence I installed Revo uninstaller and removed Dopus completely, also cleaning up the registry.
Clean install and everything was up and running as it should.
I've been using Dopus since ages but was first time I got this kind of behaviour.
Error said that there was a problem related to the install shiled installer. Tried to remove it via normal way, didn' succeed as error popped-up again during reboot.