Spring Boot returns string and not the view

Currently I'm trying to learn Spring Boot and therefore thought it would be good to write a web-application with Spring Boot Web MVC. I got stuck on a problem and have not found a solution or anything on Google which helped. I hope someone here could help me.

Goal of the project is to build a web application with Spring Boot, Thymeleaf, Bootstrap and Hibernate, all with the newest version. I've used several tutorials but there are some things which are confusing me.

I coded the views with Thymeleafs layout-dialect, but when I deploy the project on my local Tomcat and browse the index page I just get a String with "index" and not an html-file. I don't know where to look for the problem. I assume the way I try to integrate the layout-dialect or the way I try to return the view is wrong. I hope someone has an idea.

Controller:

package de.rune.flying.flying.web.controller;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.servlet.ModelAndView;

@Controller
public class UserController {

    @GetMapping ( "/" )
    public ModelAndView main ( Model model ) {

        ModelAndView modelAndView = new ModelAndView ( );
        modelAndView.setViewName ( "index" );
        return modelAndView;
    }

}

I've already tried another method (see below) but it didn't changed anything.

Other kind of method:

        @GetMapping ( "/" )
        public String main ( Model model ) {
            return "index.html";
        }

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.7.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <groupId>de.rune.flying</groupId>
    <artifactId>flying</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>war</packaging>
    <name>flying</name>

    <properties>
        <java.version>1.8</java.version>
        <bootstrap.version>4.3.1</bootstrap.version>
        <thymeleaf.dialect.version>2.4.1</thymeleaf.dialect.version>
    </properties>

    <dependencies>

        <!-- web mvc -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!-- thymeleaf -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>

        <dependency>
            <groupId>nz.net.ultraq.thymeleaf</groupId>
            <artifactId>thymeleaf-layout-dialect</artifactId>
            <version>${thymeleaf.dialect.version}</version>
        </dependency>

        <!-- test -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
        </dependency>

        <!-- dev tools -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
        </dependency>

        <!-- optional -->
        <dependency>
            <groupId>org.webjars</groupId>
            <artifactId>bootstrap</artifactId>
            <version>${bootstrap.version}</version>
        </dependency>

    </dependencies>

    <build>
        <finalName>flying</finalName>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <addResources>true</addResources>
                </configuration>
            </plugin>

            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.22.0</version>
            </plugin>
        </plugins>
    </build>

</project>

layout.html:

<!DOCTYPE html>
<html lang="de"
      xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout"
      xmlns:th="http://www.thymeleaf.org">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
    <title layout:title-pattern="$LAYOUT_TITLE - $CONTENT_TITLE">Layout</title>

    <link rel="stylesheet" th:href="@{webjars/bootstrap/4.3.1/css/bootstrap.min.css}"/>
    <link rel="stylesheet" th:href="@{/static/css/main.css}">
</head>
<body>
    <header>
        MY Header
    </header>
    <section layout:fragment="content">
        <p>Content goes here</p>
    </section>

    <footer>
        <p>My footer</p>
        <p layout:fragment="footer">Footer goes here</p>
    </footer>

    <script type="text/javascript" th:src="@{webjars/bootstrap/4.3.1/js/bootstrap.min.js}"></script>
</body>
</html>

index.html

<!DOCTYPE html>
<html lang="de"
      xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout"
      xmlns:th="http://www.thymeleaf.org"
    layout:decorate="~{layouts/layout.html}">
<head>
    <title>Index title</title>
</head>
<body>
    <section layout:fragment="content">
        <p>Index content</p>
    </section>
</body>
</html>

Let me know if some informations are missing, then I will upload them and thanks for help

edited:

project structure

Screenshot of resulted view

When I inspect the source code of the view in my browser I just see the text "index", so it seems like he just returns the string and can't find a view with the name.