error: undefined reference to 'ConvertUtf8ToKString(char const*)'

I try. to compile a lib with Android Studio. If I compile the lib with CMAKE on Linux everything went fine. With Android Studio I always get the error:

error: undefined reference to 'ConvertUtf8ToKString(char const*)'

This confuse me. The function is definitely found in the referenced CPP file.

The complete function is:

bool BsdSockServer::ReceiveAnswer(KString &refStrAnswer)
{

    static const int utf8BuffSize = UTF8_BUFF_SIZE;
    static char utf8Buff[UTF8_BUFF_SIZE];
    int res = recv(m_socket, utf8Buff, utf8BuffSize, 0);
    if(res == -1)
    {
        throw FDBException(FDB_EXC_CONNECTION_ERROR);
    }
    if(res == 0)
    {
        throw FDBException(FDB_EXC_CONNECTION_CLOSED);
    }

    if(res >= utf8BuffSize)
    {
        throw FDBException(FDB_EXC_TOO_BIG_ANSWER);
    }

    utf8Buff[res] = char(0);

    refStrAnswer = ConvertUtf8ToKString(utf8Buff); //<----- LINE

    return true;
}

The function in the KString class is:

KString ConvertUtf8ToKString(const char* szString)
{
    size_t nSrcLength = strlen(szString);
    wchar_t* wszBuffer = new wchar_t[nSrcLength + 1];

    utf2wcs(wszBuffer, szString, int(nSrcLength + 1));

    KString result(wszBuffer);
    delete[] wszBuffer;
    return result;
}

In my eyes there is everything ok. Why the NDK compile nag around? I also checked the compiler and CMAKE flags and there is nothing different. Compiler is always clang with c+11.

The important Gradle lines are:

        ndk { // these platforms cover 99% percent of all Android devices
            //abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
            abiFilters 'x86_64'
        }

        externalNativeBuild {
            cmake {
                arguments '-DANDROID_PLATFORM=android-29', '-DANDROID_TOOLCHAIN=clang', '-DANDROID_ARM_NEON=TRUE', '-DANDROID_STL=c++_static'
                cFlags '-O3', '-fsigned-char' // full optimization, char data type is signed
                cppFlags '-std=c++11', '-fsigned-char', '-fPIC'
            }
        }

Anybody can give me a tip why this compile error happen?