https load balancer with fiber is not working - go

Question

I am trying to make simple load balancer using fiber in Go. In my computer it's working fine using http.
import (
"crypto/tls"
"log"
"os"
store "intraGo/stores/session_store"
"github.com/gofiber/fiber/v2"
"github.com/gofiber/fiber/v2/middleware/proxy"
"github.com/gofiber/template/html"
)
...
if os.Getenv("CERT_FILE") == "" || os.Getenv("KEY_FILE") == "" {
// this part is working fine
app.Use(proxy.Balancer(proxy.Config{Servers: []string{"http://localhost:9000"}}))
log.Fatal(app.Listen(":" + os.Getenv("SERVER_PORT")))
} else {
app.Use(proxy.Balancer(proxy.Config{Servers: []string{"https://servername.com"}}))
cer, err := tls.LoadX509KeyPair(os.Getenv("CERT_FILE"), os.Getenv("KEY_FILE"))
if err != nil {
log.Fatal(err)
}
config := &tls.Config{Certificates: []tls.Certificate{cer}}
ln, err := tls.Listen("tcp", ":"+os.Getenv("SERVER_PORT"), config)
if err != nil {
panic(err)
}
log.Fatal(app.Listener(ln))
}
In the production envirment using https it will give me a http 500 result with the follwing text for every request. The request will never reach the server.
HostClient can't follow redirects to a different protocol, please use Client instead
I checked Go / fiber documentation but did not find anything that could help to solve this issue. I also checked if the https part (like certificates, https server) is working fine by adding this to my app:
app.get("/", (req, res) => { res.send("Hello World!"); });
This worked as expected, so I guess the problem is in the loadbalancer itself.
Found this link while trying to find the solution on the net, I am not sure, but this might be related to the problem:
https://github.com/valyala/fasthttp/issues/841