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New required entries in the application config file must be manually added to web.config for existing sites. There are two common types of entries to account for with every version upgrade: new appSettings entries and new binding redirects. These entries are documented in the general release notes for the core upgrade, as they typically apply to all products that are being upgraded. If you are upgrading your site from more than one version of Aptify, you will need to consult the documentation for those releases to understand all entries that potentially need to be added.
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Another common error is around required binding redirects, so make sure to read Read our release notes on the topic here.