CMake did not work and there was a yellow triangle at the left side of MSVC2019 kit name:

I fixed this buy changing C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat with C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsx86_arm64.bat.

In QT 5.15 I declare a class with an invokable constructor:

class TestClass : public QObject
{
    Q_OBJECT

public:

    Q_INVOKABLE TestClass() {}
};

Q_DECLARE_METATYPE(TestClass*)

register its type:

qRegisterMetaType<TestClass*>();

I get close reason: ‘Invalid UTF-8 code encountered.’ (code=1007) with QWebSocket in the following scenario:

• I connect to Binance stream wss://stream.binance.com:9443/stream with QWebSocket::open.
• Successfully receive messages for some time period (a few hours).
• Binance disconnects me when its servers load increases (because I have low-priority API) and I get close reason code=1000 that indicates a normal closure.
• Try to reconnect multiple times by calling QWebSocket::open again, but Binance closes the connection with close reason ‘Invalid UTF-8 code encountered.’ (code=1007) that indicates that an endpoint is terminating the connection because it has received data within a message that was not consistent with the type of the message (e.g., non-UTF-8 [RFC3629] data within a text message).
• Restart the application manually and connect successfully.

The difficulty of reproducing the bug is that Binance disconnects me rarely (once a day, for example) and experimentation with disconnecting the network cable from my machine does not have this effect, after I connect the cable back, QWebSocker reconnects successfully. When I disconnect the network cable I get Error: ‘Unknown error’ (code=-1), Close reason: ” (code=1000).

There was a bug in QT 5.15.2 that results in app crash on Android 5.1 and a patch:

Submodule qtbase contains modified content
diff --git a/qtbase/src/corelib/plugin/qlibrary_unix.cpp b/qtbase/src/corelib/plugin/qlibrary_unix.cpp
index a5c72f81d9..5cd21b67a4 100644
--- a/qtbase/src/corelib/plugin/qlibrary_unix.cpp
+++ b/qtbase/src/corelib/plugin/qlibrary_unix.cpp
@@ -243,10 +243,10 @@ bool QLibraryPrivate::load_sys()
             }
             if (hnd) {
                 using JniOnLoadPtr = jint (*)(JavaVM *vm, void *reserved);
-                JniOnLoadPtr jniOnLoad = reinterpret_cast<JniOnLoadPtr>(dlsym(pHnd, "JNI_OnLoad"));
+                JniOnLoadPtr jniOnLoad = reinterpret_cast<JniOnLoadPtr>(dlsym(hnd, "JNI_OnLoad"));
                 if (jniOnLoad && jniOnLoad(QtAndroidPrivate::javaVM(), nullptr) == JNI_ERR) {
                     dlclose(hnd);
-                    pHnd = nullptr;
+                    hnd = nullptr;
                 }
             }
 #endif

Looks like it was simply a typo and pHnd was replaced with hnd.

QT 5.15.3 where the bug was fixed is commercial so I applied the patch by myself (thanks to danilabagroff from stackoverflow.com):