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Ok, solved this already 🙂 (with using Pulumi's GetAppService Issue was, that when I've used the

AppServicePlanId

from Azure Portal as i posted in original post it's a little bit different than the correct one. Diff is:

From Azure Portal (which does not work)
/subscriptions/<OUR_SUB_ID>/resourceGroups/<OUR_RESOURCE_GROUP_NAME>/providers/Microsoft.Web/serverFarms/<OUR_APP_SERVICE_PLAN_NAME>

From GetAppService
/subscriptions/<OUR_SUB_ID>/resourceGroups/<OUR_RESOURCE_GROUP_NAME>/providers/Microsoft.Web/serverfarms/<OUR_APP_SERVICE_PLAN_NAME>

‼️ Note the small 'f' in

.../server*f*arms/...