Error C2662 after updating (from fmt::v8) to fmt::v10::formatter

Question

After updating my project vom fmt::v8 to fmt::v10 I get a conversion error during building in the formatter apparently. To be clear, this worked flawlessly in fmt::v8.

std::ostream& operator<<(
    std::ostream& os, MyDataType const& parameters)
{
  os << fmt::to_string(parameters);
  return os;
}

throws now

Error C2662 'auto fmt::v10::formatter<..>::format<FormatContext>(..)': cannot convert 'this' pointer from 'const fmt::v10::formatter<..>' to 'fmt::v10::formatter<..> &'

I also found this but don't understand it

template<>
struct fmt::formatter<MyDataType>
    : formatter<std::unordered_map<std::string, std::string>> {
  template<typename FormatContext>
  auto format(MyDataType const& parameters, FormatContext& ctx)
  {
    auto iter = ctx.out();
    return fmt::format_to(
        iter,
        "MyDataType("
        "{{Type, {}}})",
        static_cast<int>(parameters.type));
  }
};