delete scheduled tasks fails when task identifier is account:uid I have an action set as a scheduled task that deletes a user, that seems to be working fine:

{ "rules_delete_unregistered_user" : {
    "LABEL" : "Delete unregistered user",
    "PLUGIN" : "action set",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "USES VARIABLES" : { "user" : { "label" : "user", "type" : "user" } },
    "ACTION SET" : [ { "entity_delete" : { "data" : [ "user" ] } } ]
  }
}


and I also have a rule, that should delete this action set, but it fails:

{ "rules_user_accepted" : {
    "LABEL" : "user_accepted",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "rules_scheduler" ],
    "ON" : { "user_update" : [] },
    "IF" : [
      { "data_is" : { "data" : [ "account:field-accepted" ], "value" : "1" } }
    ],
    "DO" : [
      { "schedule_delete" : {
          "component" : "rules_delete_unregistered_user",
          "task" : [ "account:uid" ]
        }
      }
    ]
  }
}


now I do not have any idea what should I use as a task identifier to delete that action set.

EDIT:

the rule that schedules the action set:

{ "rules_create_new_user_after_registration_form" : {
    "LABEL" : "create_new_user_after_registration_form",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [ "new user based on webform" ],
    "REQUIRES" : [ "webform_rules", "rules", "php", "rules_scheduler" ],
    "ON" : { "webform_rules_submit" : [] },
    "IF" : [
      { "webform_has_id" : {
          "form_id" : [ "form-id" ],
          "selected_webform" : { "value" : { "webform-client-form-104" : "webform-client-form-104" } }
        }
      }
    ],
    "DO" : [
      { "entity_create" : {
          "USING" : {
            "type" : "user",
            "param_field_lastname" : "[data:lastname1-value-raw]",
            "param_field_firstname" : "[data:firstname-value-raw]",
            "param_name" : "[data:email-value-raw]",
            "param_mail" : "[data:email-value-raw]"
          },
          "PROVIDE" : { "entity_created" : { "new_user_request" : "new_user_request" } }
        }
      },
      { "entity_query" : {
          "USING" : {
            "type" : "user",
            "property" : "uid",
            "value" : [ "new-user-request:uid" ]
          },
          "PROVIDE" : { "entity_fetched" : { "user_fetched" : "Fetched entity" } }
        }
      },
      { "schedule" : {
          "component" : "rules_delete_unregistered_user",
          "date" : { "select" : "site:current-date", "date_offset" : { "value" : 60 } },
          "param_user" : [ "user-fetched:0" ]
        }
      }
      }
    ]
  }
}


this actually works fine.
If you go to /admin/config/workflow/rules/schedule you will see a table of your scheduled components. One of the columns in that table is the task identifier - that's the string you will have to use to delete the scheduled task. You didn't post any details related to the scheduling, so I don't know what you used as a task identifier. The task identifier is necessary because one component may be queued to execute many many times, each time with a different identifier. The component name is not unique. The task identifier is.