Was discovered while testing https://jira.atlassian.com/browse/CONFSERVER-43513
Steps to reproduce:

• install confluence on PC manually (from archive) - instructions
• try to upgrade confluence using installer - (instructions)

Actual result: there are error messages during installation (attached), but at the end Confluence is upgraded successfully

Investigation:

Found similar issues are reported in conf questions (1, 2, 3, 4)

That happens when confluence installer attempts to update keys in windows registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\${installer:installationID}