Kocaptcha broken

Environment

Installation type (OTP or From Source): Source
Pleroma version (could be found in the "Version" tab of settings in Pleroma-FE): 2.1.2
Elixir version (elixir -v for from source installations, N/A for OTP): Elixir 1.10.4 (compiled with Erlang/OTP 22)
Operating system: Ubuntu 20.04
PostgreSQL version (psql -V): psql (PostgreSQL) 11.9 (Ubuntu 11.9-1.pgdg20.04+1)

Bug description

With the following configuration:

config :pleroma, Pleroma.Captcha,
  enabled: true,
  seconds_valid: 60,
  method: Pleroma.Captcha.Kocaptcha

Attempts to load the CAPTCHA (by opening the registration page) return a 500 error, with the following appearing in server logs:

23:29:56.970 request_id=FkLo_npgZ1Y9dq4AkH0S [error] Internal server error: %ArgumentError{message: "argument error"}
23:29:56.971 [error] #PID<0.9943.186> running Pleroma.Web.Endpoint (connection #PID<0.9872.186>, stream id 1) terminated
Server: letsalllovela.in:80 (http)
Request: GET /api/pleroma/captcha
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Phoenix.HTML.Safe not implemented for %{errors: %{detail: "Internal server error"}} of type Map. This protocol is implemented for the following type(s): Decimal, Atom, BitString, Date, DateTime, Float, Integer, List, NaiveDateTime, Phoenix.HTML.Form, Time, Tuple
        (phoenix_html 2.14.2) lib/phoenix_html/safe.ex:1: Phoenix.HTML.Safe.impl_for!/1
        (phoenix_html 2.14.2) lib/phoenix_html/safe.ex:15: Phoenix.HTML.Safe.to_iodata/1
        (phoenix 1.4.17) lib/phoenix/controller.ex:776: Phoenix.Controller.__put_render__/5
        (phoenix 1.4.17) lib/phoenix/endpoint/render_errors.ex:77: Phoenix.Endpoint.RenderErrors.instrument_render_and_send/5
        (phoenix 1.4.17) lib/phoenix/endpoint/render_errors.ex:62: Phoenix.Endpoint.RenderErrors.__catch__/5
        (phoenix 1.4.17) lib/phoenix/endpoint/cowboy2_handler.ex:42: Phoenix.Endpoint.Cowboy2Handler.init/4
        (cowboy 2.8.0) /home/pleroma/lall/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
        (cowboy 2.8.0) /home/pleroma/lall/deps/cowboy/src/cowboy_stream_h.erl:300: :cowboy_stream_h.execute/3