The error message seems to indicate a transient error. Can you still reproduce this issue?

If you can, could you take a screenshot of the error message and of chrome://sync-internals?

Yes, I can reproduce constantly, I've had this issue (even though with different error message at times, since Chromium 53).

Attached screenshot with the error message and sync-internals page.

Thanks

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	sync-internals.png 141 KB View Download

For the record, the same process works with Chrome 55.0.2883.87.
The issue is specific to Chromium.

What are the keys that you're using when building Chromium?

>gn args out/Debug

What are the arguments for the following keys:
google_api_key
google_default_client_id
google_default_client_secret

Actually, I just realized that it may not be wise to put these private values o a public bug.

Please take a look at https://www.chromium.org/developers/how-tos/api-keys and use valid API keys with Chromium as otherwise signing in will not work.

I'm not building it, I'm using the Ubuntu 16.10 package (version 55.0.2883.87-0ubuntu1.16.10.1330)

Note that sync works fine with my personal google account (no SAML)

Anthony/Roger: Do you guys know of any issue of signing in with a SAML account on a chromium build?

Can I ask you for a screenshot of the chrome://signin-internals page? I'd also like to see a trace produced by opening chrome://net-internals and then reproducing the bug. The network trace is especially important if you continue to see the "Service unavailable" error message.

Attached are screenshot and dump as instructed. Thanks

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net-internals-log.json 711 KB View Download

Reposted signin-internals screenshot.

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	signin-internals.png 59.7 KB View Download

The client id for chromium-browser on Ubuntu was changed in August, which caused this bug to start happening. We have a whitelist for clients allowed to use the authentication flow used by SAML on Ubuntu; changing the client id caused the browser to be unrecognized as belonging to this whitelist. For now I think the thing to do is have them change the client id back. 

Thanks for the investigation on this issue. 
Does this means I should open a bug on the Ubuntu Chromium package?

The bug is already open here: https://bugs.launchpad.net/ubuntu/+source/chromium-browser/+bug/1637957

To update the above, the client id for chromium-browser was changed because the old key was abused, so we'll have to update our internal whitelist. In the longer term, the whitelisted endpoint will be removed and SAML auth in chrome will use a different endpoint.

Assigning to pnoland to do whitelisting, leaving at external as it won't be a chromium CL.

Sync is working again for me as of today, thanks!