Tab app for Sharepoint home page fails

I am trying to add a custom tab app for the Team's SharePoint home page, following this advice

 https://docs.microsoft.com/en-us/microsoftteams/teams-standalone-static-tabs-using-spo-sites

and also some posts scattered through these forums. The app installs along the left rail, but I get the following error: "There was a problem reaching this app". When I make a website tab within a channel using the json's contentURL, it works.

I copy my json file from the app, replacing my tenant by xxxxx. I added the extra domains based on a post I found, but that did not help. If anyone can see the problem, I would appreciate the help. Thanks.

{
"$schema": "https://developer.microsoft.com/en-us/json-schemas/teams/v1.6/MicrosoftTeams.schema.json",
"manifestVersion": "1.6",
"version": "1.0.3",
"showLoadingIndicator": true,
"id": "75d70f18-fec9-405f-98b4-f7676eedae7c",
"packageName": "com.xxxxx.home.testClass8437",
"developer": {
"name": "Steve Frank",
"websiteUrl": "https://stevefrank.org",
"privacyUrl": "https://stevefrank.org/stevehome.html",
"termsOfUseUrl": "https://stevefrank.org/stevehome.html"
},
"icons": {
"color": "color.png",
"outline": "outline.png"
},
"name": {
"short": "Home testClass8437",
"full": "Sharepoint home page for testClass8437 team"
},
"description": {
"short": "Home landing page at team Sharepoint site",
"full": "Home landing page for team testClass8437 at the associated front page for the team's Sharepoint site"
},
"accentColor": "#FFFFFF",
"staticTabs": [
{
"entityId": "HomeTestClass8437",
"name": "Home",
"contentUrl": "https://xxxxx.sharepoint.com/sites/testClass8437/_layouts/15/teamslogon.aspx?SPFX=true&dest=/sites/testClass8437",
"websiteUrl": "https://xxxxx.sharepoint.com/sites/testClass8437",
"scopes": [
"personal"
]
}
],
"permissions": [
"identity",
"messageTeamMembers"
],
"validDomains": [
"xxxxx.sharepoint.com",
"resourceseng.blob.core.windows.net",
"*.sharepoint-df.com",
"teams.microsoft.com",
"*.login.microsoftonline.com",
"*.stevefrank.org"
],
"webApplicationInfo": {
"id": "00000003-0000-0ff1-ce00-000000000000",
"resource": "https://xxxxx.sharepoint.com"
}
}