Enter about:sync-log in the URL bar and check, if there any recent error files.

1598693918722 Sync.Service INFO Starting sync at 2020-08-29 12:38:38 in browser session sOgvsaBVulfH 1598693918722 Sync.Service DEBUG In sync: should login. 1598693918722 Sync.Service INFO User logged in successfully - verifying login. 1598693918724 FirefoxAccounts TRACE not checking freshness of profile as it remains recent 1598693918724 Sync.BrowserIDManager DEBUG unlockAndVerifyAuthState already has (or can fetch) sync keys 1598693918724 Sync.Status DEBUG Status.login: success.login => success.status_ok 1598693918724 Sync.Status DEBUG Status.service: success.status_ok => error.login.failed 1598693918724 Sync.Service DEBUG Fetching unlocked auth state returned success.status_ok 1598693918724 FirefoxAccounts TRACE not checking freshness of profile as it remains recent 1598693918725 Sync.BrowserIDManager INFO Getting keys 1598693918725 Sync.BrowserIDManager INFO Getting an assertion from: https://MySyncServer/token/1.0/sync/1.5 1598693918725 Sync.BrowserIDManager DEBUG Getting a token using OAuth 1598693918725 FirefoxAccounts DEBUG getAccessToken enter 1598693918725 FirefoxAccounts DEBUG getOAuthToken enter 1598693918787 FirefoxAccounts TRACE finished write of json user data - took: 70 1598693918787 FirefoxAccounts DEBUG writing secure storage: ["kSync","kXCS","kExtSync","kExtKbHash"] 1598693918787 FirefoxAccounts TRACE starting write of user data to the login manager 1598693918789 FirefoxAccounts TRACE finished write of user data to the login manager 1598693918790 Services.Common.RESTRequest DEBUG POST request to https://api.accounts.firefox.com/v1/oauth/token 1598693918790 Services.Common.RESTRequest DEBUG POST Length: 127 1598693919026 Services.Common.RESTRequest DEBUG POST https://api.accounts.firefox.com/v1/oauth/token 200 1598693919026 Hawk DEBUG (Response) /oauth/token: code: 200 - Status text: OK 1598693919027 Hawk DEBUG Clock offset vs https://api.accounts.firefox.com/v1: -1026 1598693919027 FirefoxAccounts DEBUG _updateAccountData with items: ["oauthTokens"] 1598693919027 FirefoxAccounts DEBUG writing plain storage: ["email","sessionToken","uid","verified","device","oauthTokens","profileCache"] 1598693919027 FirefoxAccounts TRACE starting write of json user data: ["email","sessionToken","uid","verified","device","oauthTokens","profileCache"] 1598693919028 FirefoxAccounts DEBUG Fetching scopedKeys data for https://identity.mozilla.com/apps/oldsync 1598693919034 FirefoxAccounts TRACE finished write of json user data - took: 6 1598693919034 FirefoxAccounts DEBUG writing secure storage: ["kSync","kXCS","kExtSync","kExtKbHash"] 1598693919034 FirefoxAccounts TRACE starting write of user data to the login manager 1598693919042 FirefoxAccounts TRACE finished write of user data to the login manager 1598693919047 Services.Common.RESTRequest DEBUG POST request to https://api.accounts.firefox.com/v1/account/scoped-key-data 1598693919047 Services.Common.RESTRequest DEBUG POST Length: 84 1598693919277 Services.Common.RESTRequest DEBUG POST https://api.accounts.firefox.com/v1/account/scoped-key-data 200 1598693919278 Hawk DEBUG (Response) /account/scoped-key-data: code: 200 - Status text: OK 1598693919278 Hawk DEBUG Clock offset vs https://api.accounts.firefox.com/v1: -278 1598693919280 FirefoxAccounts DEBUG _updateAccountData with items: ["scopedKeys"] 1598693919280 FirefoxAccounts DEBUG writing plain storage: ["email","sessionToken","uid","verified","device","oauthTokens","profileCache"] 1598693919280 FirefoxAccounts TRACE starting write of json user data: ["email","sessionToken","uid","verified","device","oauthTokens","profileCache"] 1598693919281 Services.Common.TokenServerClient DEBUG Beginning OAuth token exchange: https://MySyncServer/token/1.0/sync/1.5 1598693919282 Services.Common.RESTRequest DEBUG GET request to https://MySyncServer/token/1.0/sync/1.5 1598693919286 FirefoxAccounts TRACE finished write of json user data - took: 6 1598693919286 FirefoxAccounts DEBUG writing secure storage: ["kSync","kXCS","kExtSync","kExtKbHash","scopedKeys"] 1598693919286 FirefoxAccounts TRACE starting write of user data to the login manager 1598693919293 FirefoxAccounts TRACE finished write of user data to the login manager 1598693919327 Services.Common.RESTRequest DEBUG GET https://MySyncServer/token/1.0/sync/1.5 401 1598693919327 Services.Common.TokenServerClient DEBUG Got token response: 401 1598693919327 Services.Common.TokenServerClient INFO Server-reported error: {"location":"body","name":"","description":"Unsupported"} 1598693919328 Services.Common.RESTRequest DEBUG POST request to https://api.accounts.firefox.com/v1/oauth/destroy 1598693919328 Services.Common.RESTRequest DEBUG POST Length: 969 1598693919329 FirefoxAccounts DEBUG _updateAccountData with items: ["oauthTokens"] 1598693919329 FirefoxAccounts DEBUG writing plain storage: ["email","sessionToken","uid","verified","device","oauthTokens","profileCache"] 1598693919329 FirefoxAccounts TRACE starting write of json user data: ["email","sessionToken","uid","verified","device","oauthTokens","profileCache"] 1598693919329 Sync.BrowserIDManager WARN Token server returned 401, refreshing certificate and retrying token fetch

During OAuth token exchange my SyncServer gives a 401 response. Maybe FF80 is not compatible with Firefox Sync Server?

Also in the new Android Firefox 79 I cannot sync with a private Sync server. When I try to open about:config I get the error in the image and the following link:

resource://android/assets/low_and_medium_risk_error_pages.html?&title=Cannot+Complete+Request&button=Try+Again&description=%3Cp%3EAdditional+information+about+this+problem+or+error+is+currently+unavailable.%3C%2Fp%3E&image=&showSSL=false&badCertAdvanced=Advanced%E2%80%A6&badCertTechInfo=%3Clabel%3ESomeone+could+be+trying+to+impersonate+the+site+and+you+should+not+continue.%3C%2Flabel%3E+%3Cbr%3E%3Cbr%3E+%3Clabel%3EWeb+sites+prove+their+identity+via+certificates.+Firefox+does+not+trust+%3Cb%3Eabout%3Aconfig%3C%2Fb%3E+because+its+certificate+issuer+is+unknown%2C+the+certificate+is+self-signed%2C+or+the+server+is+not+sending+the+correct+intermediate+certificates.%3C%2Flabel%3E&badCertGoBack=Go+Back+%28Recommended%29&badCertAcceptTemporary=Accept+the+Risk+and+Continue

With this issue, go to this question => /questions/1301042.

With this issue, go to this question => /questions/1301042.

Yes. Now I can set custom sync server to Android. When I try to sync I get "Last synced: Jan 1, 1970" I have tried loading the latest docker to run sync server, but still gives 401

I suppose that the best option for you is to file a bug on https://github.com/mozilla-mobile/fenix/issues

Looks like that Sync server and FF80 have problems. https://github.com/mozilla-services/syncserver/issues/225