The TL;DR Version:
If you are using any static method from the Runtime QML API (e.g., ArcGISRuntimeEnvironment.createObject(), SpatialReference.createWgs84(), GeometryEngine.project(), etc.), with version 100.8 or older, those methods will not work with Qt 5.15. The most reliable solution in this case is to use an older version of Qt, such as 5.12, 5.13, or 5.14. For upcoming releases of the software (version 100.9 and beyond), the majority of cases will begin to work again with no code changes, and a handful of cases will require a minor code change (see below). This issue does not affect the C++ API.
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