There is currently an issue with Masto.host Object Storage provider (OVH) that is causing media uploads to fail.
The information I got just now on a phone call with OVH is that it's a network issue and they working on it.
Sorry for the inconvenience and will update as soon as I know more.
@mastohost good opportunity to examine https://status.masto.host/ in the mean time? "All services are online."
@NeatNit Completely forgot to do that. Just added a note there.