https://github.com/frappe/frappe_docker/blob/main/docs/single-server-example.md

Line 142 - sed -i 's/SITES=erp.example.com/SITES=`one.example.com`,`two.example.com`/g' ~/gitops/erpnext-one.env

That syntax worked in Traefik v2.11, but it no longer works in v3.

Starting with Traefik v3, all matchers (including Host) accept only one value and you must combine multiple hosts with logical OR:
Host(\a.example.com) || Host(b.example.com). This change is documented in the v2→v3 migration guidance and called out in a GitHub issue where the old form triggers: “unexpected number of parameters; got 2, expected one of [1]”.

traefik.http.routers.erp.rule: "Host(one.example.com) || Host(two.example.com)"

Tip: keep the whole rule in double quotes and the matchers’ values in backticks; that matches Traefik’s examples and avoids YAML parsing hiccups.