A few more data points:

1) My local sync of the R70 branch failed with similar "Couldn't find remote ref" errors

2) amd64-generic-paladin-tryjob fails: https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8936164491922219104

3) coral-paladin-tryjob makes it past the sync stage: https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8936094716733644320

tatl, tael, and amd64-generic are all external build configs using the external manifest. Maybe there's a problem with that manifest?

Found it. I reran my local sync with -j1 so that I actually got the error message next to the failing repo:

Fetching project chromiumos/platform/touch_noise_filter
fatal: Couldn't find remote ref refs/heads/release-R70-11021.B

This was dropped from the manifest here: https://crrev.com/c/1187394

So this somehow made it before the branch was created (since that repo lacks an R70 branch), but also didn't make it because it's not in the manifest branch for R70: https://chromium.googlesource.com/chromiumos/manifest/+log/release-R70-11021.B/

I'll just remove it from R70 then...

+vapier FYI and Geo for R70

Requesting merge to 70 for https://crrev.com/c/1187394

We need this to fix the R70 builders for any external config builders.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/manifest/+/e1a7bec29669d9132efb27ab13a41916cb8d3657

commit e1a7bec29669d9132efb27ab13a41916cb8d3657
Author: Mike Frysinger <vapier@chromium.org>
Date: Fri Sep 07 16:18:36 2018

touch_noise_filter: drop

The only package that used this repo has been removed in CL:1183638.

BUG= chromium:881185 
TEST=tatl/tael tryjobs on R70 succeed

Reviewed-on: https://chromium-review.googlesource.com/1187394
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit 4a9048b14927651ab544ab3043d2f226f6a2d817)

Conflicts:
	full.xml - trivial conflict due to formatting change

Also update commit message for R70.

Change-Id: I8cba4a52d7f02e33c006e15fa5a2c90d64b1ff4f
Reviewed-on: https://chromium-review.googlesource.com/1211697
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>

[modify] https://crrev.com/e1a7bec29669d9132efb27ab13a41916cb8d3657/full.xml

Not fixed, another repo wasn't branched.

Log: https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8936024759285899504

CL at https://crrev.com/c/1213804

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/manifest/+/49cc021b79977c89da4c37e4ae989df07b7af069

commit 49cc021b79977c89da4c37e4ae989df07b7af069
Author: Stephen Barber <smbarber@chromium.org>
Date: Fri Sep 07 19:21:30 2018

manifest: drop logstock from R70

logstock was added to the internal manifest after branch, so it wasn't
branched for R70. Remove it from the external manifest to stop
sync errors.

BUG= chromium:881185 
TEST=tatl/tael-full tryjobs

Change-Id: I7ae576ef0bb25987bc776ae70bda153a467b42fd
Reviewed-on: https://chromium-review.googlesource.com/1213804
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>

[modify] https://crrev.com/49cc021b79977c89da4c37e4ae989df07b7af069/full.xml

Now getting past the Sync stage.

https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8936015878254097728