The WASM networking code crashes on us. Especially in more complex setups (5.14 also did not work without modifications from my side):

QNetworkReplyWasmImplPrivate::doSendRequest()
does not correctly handle headers.

    if (headersData.count() > 0) {
        const char* customHeaders[arrayLength];
        int i = 0;
        for (i; i < headersData.count() * 2; (i = i + 2)) {
            customHeaders[i] = headersData[i].constData();
            customHeaders[i + 1] = request.rawHeader(headersData[i]).constData();
        }
        customHeaders[i] = nullptr;
        attr.requestHeaders = customHeaders;
    }

This code cannot work as headerData is accessed with i which can go twice as high as headerData.count()

I'm also not sure whether its save to define customHeaders like this, especially not in the context of emscripten, but I assume the headers need to be allocated somehow.

    if (outgoingData) { // data from post request
        // handle extra data
        QByteArray extraData;
        extraData = outgoingData->readAll(); // is there a size restriction here?
        if (!extraData.isEmpty()) {
            attr.requestData = extraData.constData();
            attr.requestDataSize = extraData.size();
        }
    }

This code also looks dangerous and unlikely to reliably work. extraData is going out of scope, I'm not sure the constData pointer object will survive that, but it non-WASM environments this is very likely to crash.

    m_fetch = emscripten_fetch(&attr, request.url().toString().toUtf8());

Same here. Its not clear whether emscripten_fetch copies the provided url. If not this is likely to crash.

I would provide a fix, but currently we have a lot of remaining issues which are not fixed, by the above changes (except for a simple test case).