I have created what I believe to be a valid installer created as package type of partialupgrade. It specifies the UID of the already installed target of the upgrade in the header and contains all other required directives. On some Series60 and some UIQ devices, installation of this fails with "Install error". The failing devices are OS 9.1, but the installer works on a 9.1 E62 that I have. I have seen the failure myself on a SonyEricsson 990i and a Nokia N95, and it has been reported by others on a N80.
The odd thing is that if I remove a particular file from the upgrade, the install works.Only removing that file makes the install work. That file was installed with the same name and SID by the original installer.
The only other out of the ordinary thing I can think of here is that the original installer contained several nested installers, and the upgrade installer targets one of those nested installers.
P.S. - I posted this issue on the Symbian Developer Network, and tech support there passed the failing test case on to Symbian development, who said they saw nothing wrong with the base installer or the upgrade installer. I gave them all the binaries needed to recreate the installers and sign them themselves, so I can do the same here if necessary if Nokia tech support requests them.