admin login failed

[la-Forge-DE]

I work as an admin in the public sector in a small administration in the country.
Now I was repeatedly asked for my own cloud. Now I'm in the process and have decided on OpenCloud.
I have installed a Debia Trixi as a virtual machine. Set up Docker Compose according to the instructions. The ‘Hello World’ also works.
Then created the directory ‘/opencloud’ and provided everything with chown -R 1000:1000.
With ‘git clone’ into this directory and configured the .env file.
After running ‘docker compose up -d’ I wanted to call up the domain in the browser. the login screen appeared. I was able to enter the name <admin> and the self-assigned password.
But all I could see was a window saying ‘not logged in’ 'log in again' I should contact the administrator.

In the .env file is
#INSECURE=true commented out
the entry:
#TRAEFIK_ACME_CASERVER= ... also commented out according to the instructions.

_TRAEFIK_DOMAIN=traefik.meinnetz.local
OC_DOMAIN=cloud.meinnetz.local
COLLABORA_DOMAIN=collabora.meinnetz.local
WOPISERVER_DOMAIN=wopiserver.meinnetz.local

OC_CONFIG_DIR=/opencloud/config
OC_DATA_DIR=/opencloud/data_
Was set up according to the instructions.

The request #767 is similar to my problem but not the same. Nevertheless, I followed the instructions there.

What can I do to get a successful login?

[AlexAndBear]

Can you scan the logs 'docker compose logs -f' for any suspicious output ?