When interchange is enabled the “Default Importer” is disable. Sorry I wasn’t clear.
If you want to use the Importer your used to, you need to disable Interchange.
Just because you haven’t change any settings doesn’t mean that Epic doesn’t change settings between 5.4 and 5.5. Since Fab.com Plugin didn’t exist in 5.4 and is now turn on by default and requires Interchange, your project settings have change.
Any time the minor version number changes this means breaking changes. This mean you need to revisit your projects settings and plugins with each upgrade.