Traefik throwing error 502 with Kotlin Spring Boot application

Ok so I'm facing an annoying issue with Traefik in Docker: it constantly throws an error 502: Bad Gateway when I'm trying to reach my HTTP service.
All I'm trying to do is add it as a simple load balancer to build 3 replicas of my Spring Boot app. The app works well when I remove anything concerning Traefik.

My Spring Boot Controller maps /api/v1 endpoint, which is working well without load balancing. I'm trying to map this same endpoint to Traefik, which is using port :80.
Dashboard at :8080 works just fine, I have no error whatsoever about my configuration mentioned in it; only problems when I'm reaching anything under :80.

I have to mention that this exact same configuration (docker-compose, traefik conf, postgres db, and spring boot app) DOES WORK fine with the exact same docker-compose.yml AND traefik.yml, which makes my problem even weirder. Maybe the issue comes from the fact that my Spring Boot app is made of Kotlin and not the one where it's working?. Anyway, here are my files:

# Dockerfile (default one generated)
FROM maven:latest

WORKDIR /usr/src/app

COPY . /usr/src/app
RUN mvn package

ENV PORT 5000
EXPOSE $PORT
CMD [ "sh", "-c", "mvn -Dserver.port=${PORT} spring-boot:run" ]

# docker-compose.yml (works as-is in my other app)
version: "3.8"
services:
    project:
        build: .
        deploy:
            replicas: 3
        labels:
            - "traefik.http.routers.project.rule=PathPrefix(`/api/v1`)" # same endpoint as in my Spring controller
            - "traefik.http.middlewares.project.stripprefix.forceSlash=false"
            - "traefik.http.routers.project.middlewares=project@docker"

    traefik:
        image: traefik:latest
        restart: unless-stopped
        ports:
            - "80:80"
            - "8080:8080"
        volumes:
            - /var/run/docker.sock:/var/run/docker.sock
            - ./traefik.yml:/etc/traefik/traefik.yml

    postgres:
        image: postgres:latest
        container_name: postgres
        ports:
            - "5432:5432"
        environment:
            POSTGRES_PASSWORD: example

#    rabbitmq:
#        image: rabbitmq:management-alpine
#        container_name: rabbitmq
#        ports:
#            - "5672:5672" # AMQP
#            - "15672:15672" # HTTP (Management)

# traefik.yml
global:
    checkNewVersion: true
    sendAnonymousUsage: false

providers:
    docker: {}

log:
    level: DEBUG # temporarily

api:
    insecure: true

entryPoints:
    insecure:
        address: ":80"
    secure:
        address: ":443"

# src/main/resources/application.yml
spring:
    sql.init.mode: always
    datasource:
        url: jdbc:postgresql://localhost:5432,postgres:5432/postgres
        username: postgres
        password: example
    jpa:
        open-in-view: false

<!-- 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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>project</artifactId>
    <version>1.0.0</version>
    <packaging>jar</packaging>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.6.4</version>
        <relativePath/>
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>17</java.version>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
        <kotlin.version>1.6.10</kotlin.version>
    </properties>

    <dependencies>
        <!-- Kotlin -->
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-reflect</artifactId>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jdk8</artifactId>
        </dependency>

        <!-- Spring Boot -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-amqp</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <!-- PostgreSQL -->
        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
        </dependency>

    </dependencies>

    <build>
        <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <configuration>
                    <args>
                        <arg>-Xjsr305=strict</arg>
                    </args>
                    <compilerPlugins>
                        <plugin>spring</plugin>
                    </compilerPlugins>
                </configuration>
                <dependencies>
                    <dependency>
                        <groupId>org.jetbrains.kotlin</groupId>
                        <artifactId>kotlin-maven-allopen</artifactId>
                        <version>${kotlin.version}</version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </build>

</project>

I'm using JDK 17, and docker-compose up --build to start everything. Thanks!