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Problem to get through to a container port

Hi everybody,

I have a problem with one of my Docker containers. It provides two web application services, RStudio Server and R Shiny Server. It is started with

docker run -d -p 127.0.0.1:3840:3838 -p 127.0.0.1:8790:8787 -v /var/www/shiny-server/:/srv/shiny-server/ --name rservers --restart always rservers

and a docker ps gives

# docker ps
CONTAINER ID        IMAGE                        COMMAND                  CREATED             STATUS              PORTS                                                NAMES
1326f5f9eac7        rservers                     "/install/start-serv…"   16 hours ago        Up 16 hours         127.0.0.1:3840->3838/tcp, 127.0.0.1:8790->8787/tcp   rservers
136ea0f63229        chockemeyer/rservers:3.5.1   "/install/start-serv…"   19 months ago       Up 2 weeks          127.0.0.1:3839->3838/tcp, 127.0.0.1:8788->8787/tcp   rservers-3.5.1

Now I can reach port 3840 without problems but not port 8790. If I telnett to it I receive

# telnet localhost 8790
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Connection closed by foreign host.

This means the porrt is listened but not (correctly) connected to the service. If I do the same telnet call (to port 8787) within the container, everything works fine.
As you can see from the above docker ps output, I have a second, parallel container running (with just a different R version) where everything works fine.
Host system and container are both Ubuntu 18.04, Docker version 19.03.8, build afacb8b7f0.

I woud be grateful for any ideas as I have run out of them…