App Validation failing for no clear reason

We're trying to get our (very simple) Teams App listed on the app store, but during the App Validation we keep getting the same error: "Unable to upload the manifest.zip file in MS Teams." as seen below:

Downloading the report doesn't give any more information, it only lists the only successful validation step ("BotID should be registered") and the error doesn't appear there.

I've validated the manifest.json file against the schema, tried reuploading the same manifest to a different org (creating a new app and bot) - validation fails there too.

Does anyone have any pointers? Here is my (anonymized) manifest file:

{
  "$schema": "https://developer.microsoft.com/en-us/json-schemas/teams/v1.19/MicrosoftTeams.schema.json",
  "manifestVersion": "1.19",
  "version": "1.1.1",
  "id": "[redacted]",
  "developer": {
    "name": "[redacted]",
    "websiteUrl": "https://www.[redacted].com",
    "privacyUrl": "[redacted]",
    "termsOfUseUrl": "[redacted]"
  },
  "icons": {
    "color": "color.png",
    "outline": "outline.png"
  },
  "name": {
    "short": "[redacted]",
    "full": "[redacted]"
  },
  "description": {
    "short": "[redacted]",
    "full": "[redacted]"
  },
  "accentColor": "#4A3AFF",
  "bots": [
    {
      "botId": "[redacted]",
      "scopes": ["personal"],
      "supportsFiles": false,
      "isNotificationOnly": false,
      "commandLists": [
        {
          "scopes": ["personal"],
          "commands": [
            {
              "title": "help",
              "description": "Show what this bot does and how to connect your account"
            },
            {
              "title": "connect",
              "description": "Get a fresh link to (re-)connect your account"
            }
          ]
        }
      ]
    }
  ],
  "staticTabs": [
    { "entityId": "conversations", "scopes": ["personal"] },
    { "entityId": "about", "scopes": ["personal"] }
  ],
  "permissions": ["identity", "messageTeamMembers"],
  "validDomains": ["app.[redacted].com", "auth.[redacted].com"]
}