504 error when juju deploy

I tried:

juju deploy prometheus2
juju deploy prometheus-alertmanager
juju relate prometheus-alertmanager:alertmanager-service prometheus2:alertmanager-service

.....

juju debug-log

2021-04-07 11:20:29 INFO juju.downloader download.go:174 download verified ("cs:prometheus-alertmanager-16")
2021-04-07 11:20:31 INFO juju.worker.uniter uniter.go:314 hooks are retried true
2021-04-07 11:20:31 INFO juju.worker.uniter.storage resolver.go:125 initial storage attachments ready
2021-04-07 11:20:31 INFO juju.worker.uniter resolver.go:147 found queued "install" hook
2021-04-07 11:22:32 INFO juju-log Reactive main running for hook install
2021-04-07 11:22:32 ERROR juju-log Unable to find implementation for relation: peers of alertmanager-cluster
2021-04-07 11:22:33 INFO juju-log Initializing Snap Layer
2021-04-07 11:22:37 WARNING install All snaps up to date.
2021-04-07 11:22:37 INFO juju-log Invoking reactive handler: reactive/alertmanager.py:38:install_packages
2021-04-07 11:22:37 WARNING install ERROR could not download resource: HTTP request failed: Get https://192.168.2.224:17070/model/583f3d6c-4717-48fb-842c-d73a5e554e9d/units/unit-prometheus-alertmanager-0/resources/prometheus-alertmanager: resource#prometheus-alertmanager/prometheus-alertmanager not found
2021-04-07 11:22:37 INFO juju-log Installing prometheus-alertmanager from store
2021-04-07 11:36:10 ERROR juju-log Installation failed cmd="['snap', 'install', '--channel=stable', 'prometheus-alertmanager']" returncode=1 output="b'2021-04-07T11:24:27Z INFO Waiting for automatic snapd restart...\nerror: cannot perform the following tasks:\n- Download snap "prometheus-alertmanager" (11) from channel "stable" (received an unexpected http response code (504) when trying to download https://canonical-lcy01.cdn.snapcraft.io/download-origin/canonical-lgw01/BVP0M5TocZED93bvyOvxpnIbkCsauLJe_11.snap?interactive=1&token=1617807600_3b3dd090d1aa2e6fd0d42035920ec6053fb952fc)\n'"

...
...

2021-04-07 11:39:20 WARNING install ERROR could not download resource: HTTP request failed: Get https://192.168.2.224:17070/mo
del/583f3d6c-4717-48fb-842c-d73a5e554e9d/units/unit-prometheus-alertmanager-0/resources/prometheus-alertmanager: resource#prom
etheus-alertmanager/prometheus-alertmanager not found

The charm is stuck in "maintenance" for about 20 minutes, then it somehow seems to work and installation proceeds. I'm not sure why this happens or why it suddenly breaks free and installs but clearly something goes sideways here.