What seems to happen is that the post-install script wants to load a kernel-module that already is loaded (or something like that, I can’t remember) and returns a non-zero exit code even though there is not really a problem.
But the error keeps being reported every time you install packages. To fix this you can simply mark the package as “installed” and all is fine.
The problem is that the post-install script reports an error that really is not an error.
But this causes the error-message that you’ll then see again and again every time you install a package as for the opkg-system the installation of the bluetooth module has not completed yet.
To get rid of this (wrong) error message you simply do a “opkg mark installed kmod-bluetooth”.
Then opkg assumes the installation is complete and the error messagee goes away. And bluetooth works as there was never really a problem, so everyting is fine: Working bluetooth and no more error-messages.
This is of course only a work-around for a problem that should properly be fixed in the post-install script of this package.