Add troubleshooting guide

README.md

@@ -129,3 +129,35 @@ emails.
 - @monitoring supervisorctl -> take control over the monitoring system
 - @monitoring help -> show this message
 ```
+
+### Troubleshooting
+
+#### Database connection needed
+
+The monitoring processes talk to the KM3NeT Oracle DB service and need an
+valid session cookie. The monitoring servers of the ORCA and ARCA shore stations
+are whitelisted, but if you run the system on other machines, you need to provide
+the cookie string for that specific machine. To get the cookie string, run the
+monitoring system with `docker-compose up -d` and connect to the backend with
+
+    # docker exec -it monitoring_backend_1 bash
+
+To get a session cookie, query the database however you like, e.g.
+
+    # streamds get detectors
+
+It will ask you for your KM3NeT (external) credentials and the required
+cookie value is the last column in the file `~/.km3netdb_cookie`
+
+    # cat ~/.km3netdb_cookie
+    .in2p3.fr	TRUE	/	TRUE	0	sid	_tgal_131.42.5.23_6d132a51d884b22b2ba861a8847346c
+
+Create a new environment variable in the `.env` file on the host system (not in the Docker
+container!) with the following entry (of course with your cookie string):
+
+    KM3NET_DB_COOKIE=_tgal_131.42.5.23_6d132a51d884b22b2ba861a8847346c
+
+and restart the whole monitoring system with
+
+    docker-compose down && docker-compose up -d
+