Server Dashboard SSL Support
Added a simple if-else statement that checks TLSOnly mode enabled. If enabled, serves the dashboard in HTTPS mode. Otherwise, serves the dashboard in HTTP mode. 
If TLSCertFile and TLSKeyFile are not used in config but TLSOnly is enabled, the HTTPS dashboard does not loads.

Tested on Debian Linux and Windows Server 2012.
server/dashboard.go
// Copyright 2017 fatedier, fatedier@gmail.com
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package server

import (
	"net"
	"net/http"
	"net/http/pprof"
	"time"

	"github.com/fatedier/frp/assets"
	frpNet "github.com/fatedier/frp/pkg/util/net"

	"github.com/gorilla/mux"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

var (
	httpServerReadTimeout  = 60 * time.Second
	httpServerWriteTimeout = 60 * time.Second
)

func (svr *Service) RunDashboardServer(address string) (err error) {
	// url router
	router := mux.NewRouter()
	router.HandleFunc("/healthz", svr.Healthz)

	// debug
	if svr.cfg.PprofEnable {
		router.HandleFunc("/debug/pprof/cmdline", pprof.Cmdline)
		router.HandleFunc("/debug/pprof/profile", pprof.Profile)
		router.HandleFunc("/debug/pprof/symbol", pprof.Symbol)
		router.HandleFunc("/debug/pprof/trace", pprof.Trace)
		router.PathPrefix("/debug/pprof/").HandlerFunc(pprof.Index)
	}

	subRouter := router.NewRoute().Subrouter()

	user, passwd := svr.cfg.DashboardUser, svr.cfg.DashboardPwd
	subRouter.Use(frpNet.NewHTTPAuthMiddleware(user, passwd).Middleware)

	// metrics
	if svr.cfg.EnablePrometheus {
		subRouter.Handle("/metrics", promhttp.Handler())
	}

	// api, see dashboard_api.go
	subRouter.HandleFunc("/api/serverinfo", svr.APIServerInfo).Methods("GET")
	subRouter.HandleFunc("/api/proxy/{type}", svr.APIProxyByType).Methods("GET")
	subRouter.HandleFunc("/api/proxy/{type}/{name}", svr.APIProxyByTypeAndName).Methods("GET")
	subRouter.HandleFunc("/api/traffic/{name}", svr.APIProxyTraffic).Methods("GET")

	// view
	subRouter.Handle("/favicon.ico", http.FileServer(assets.FileSystem)).Methods("GET")
	subRouter.PathPrefix("/static/").Handler(frpNet.MakeHTTPGzipHandler(http.StripPrefix("/static/", http.FileServer(assets.FileSystem)))).Methods("GET")

	subRouter.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		http.Redirect(w, r, "/static/", http.StatusMovedPermanently)
	})

	server := &http.Server{
		Addr:         address,
		Handler:      router,
		ReadTimeout:  httpServerReadTimeout,
		WriteTimeout: httpServerWriteTimeout,
	}
	if address == "" || address == ":" {
		address = ":http"
	}
	ln, err := net.Listen("tcp", address)
	if err != nil {
		return err
	}

	go server.Serve(ln)
	return
}

// Copyright 2017 fatedier, fatedier@gmail.com
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package server

import (
	"crypto/tls"
	"net"
	"net/http"
	"net/http/pprof"
	"time"

	"github.com/fatedier/frp/assets"
	frpNet "github.com/fatedier/frp/pkg/util/net"

	"github.com/gorilla/mux"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

var (
	httpServerReadTimeout  = 60 * time.Second
	httpServerWriteTimeout = 60 * time.Second
)

func (svr *Service) RunDashboardServer(address string) (err error) {
	// url router
	router := mux.NewRouter()
	router.HandleFunc("/healthz", svr.Healthz)

	// debug
	if svr.cfg.PprofEnable {
		router.HandleFunc("/debug/pprof/cmdline", pprof.Cmdline)
		router.HandleFunc("/debug/pprof/profile", pprof.Profile)
		router.HandleFunc("/debug/pprof/symbol", pprof.Symbol)
		router.HandleFunc("/debug/pprof/trace", pprof.Trace)
		router.PathPrefix("/debug/pprof/").HandlerFunc(pprof.Index)
	}

	subRouter := router.NewRoute().Subrouter()

	user, passwd := svr.cfg.DashboardUser, svr.cfg.DashboardPwd
	subRouter.Use(frpNet.NewHTTPAuthMiddleware(user, passwd).Middleware)

	// metrics
	if svr.cfg.EnablePrometheus {
		subRouter.Handle("/metrics", promhttp.Handler())
	}

	// api, see dashboard_api.go
	subRouter.HandleFunc("/api/serverinfo", svr.APIServerInfo).Methods("GET")
	subRouter.HandleFunc("/api/proxy/{type}", svr.APIProxyByType).Methods("GET")
	subRouter.HandleFunc("/api/proxy/{type}/{name}", svr.APIProxyByTypeAndName).Methods("GET")
	subRouter.HandleFunc("/api/traffic/{name}", svr.APIProxyTraffic).Methods("GET")

	// view
	subRouter.Handle("/favicon.ico", http.FileServer(assets.FileSystem)).Methods("GET")
	subRouter.PathPrefix("/static/").Handler(frpNet.MakeHTTPGzipHandler(http.StripPrefix("/static/", http.FileServer(assets.FileSystem)))).Methods("GET")

	subRouter.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		http.Redirect(w, r, "/static/", http.StatusMovedPermanently)
	})

	server := &http.Server{
		Addr:         address,
		Handler:      router,
		ReadTimeout:  httpServerReadTimeout,
		WriteTimeout: httpServerWriteTimeout,
	}
	ln, err := net.Listen("tcp", address)
	if err != nil {
		return err
	}

	if svr.cfg.DashboardTLSMode {
		cert, err := tls.LoadX509KeyPair(svr.cfg.DashboardTLSCertFile, svr.cfg.DashboardTLSKeyFile)
		if err != nil {
			return err
		}
		tlsCfg := &tls.Config{
			Certificates: []tls.Certificate{cert},
		}
		ln = tls.NewListener(ln, tlsCfg)
	}
	go server.Serve(ln)
	return
}

I think it's a temporary error. Retry the test can fix it.