Seafile pro caddy SEAFILE_SERVER_PROTOCOL

Guests that are trying to upload files through the uplink link web interface are confronted with a “Network error”

Mixed Content: The page at 'https://<snip>/u/d/<snip>/' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://<snip>/seafhttp/upload-api/<snip>?ret-json=1'. This request has been blocked; the content must be served over HTTPS.
i.send @ commons.c26c5363.ac4e586840ba.js:2
GET https://<snip>/api/v2.1/upload-links/<snip>/upload/
<
{upload_link: "http://<snip>/seafhttp/upload-api/<snip>"}

Why is it returning a http?

Conform documentations i have set
SEAFILE_SERVER_HOSTNAME=myhostname.com
SEAFILE_SERVER_PROTOCOL=http

Yes http because else CADDY will not work because caddy and seafile ARE NOT the SSL handler. Traefik is doing the SSL offloading. Setting SEAFILE_SERVER_PROTOCOL to https will brick the caddy config and renders the server unaccessible.

Please help me with this issue.

If you use Traefik as reverse proxy, you should remove CADDY.

You can check this document: Use other reverse proxy - Seafile Admin Manual