a) The installer uses a package called uimserverpackages.zip. If the install files are always downloaded to the same location, then uimserverpackages.zip will be renamed to uimserverpackages(1).zip and the installer ends up using the pre-existing uimserverpackages.zip which is 9.0.2. The install works but UIM doesn't. So make sure you download uimserverpackages.zip for UIM v9.20 and don't overwrite it.
b) Make sure you shut down any anti-virus software or firewalls that could interfere with upgrading/installation before proceeding.