Unity cloud build fails with no errors or warnings

But a freshly cloned copy of the game builds fine locally.

Here are my logs:

Running Prebuild steps
In quiet period, build will start momentarily...
Agent pid 5753
Successfully uninstalled bvr-2.0.9
Removing bvr
Removing bvr_runner
Successfully uninstalled bvr-2.0.18
bvr 2.0.22
bvr-api 0.3.8
prebuildstatus finished successfully.
All identities removed.
BVR: SCM Checkout
GIT: Checkout project...
GIT: Checking out git@bitbucket.org:spottedzebra/questbert.git/master
Cloned in 2 seconds.
GIT: Checkout operation complete.
BVR: SCM Checkout completed in 3 seconds.
Calculated Workspace Size: 7.03 MiB
PATH | SIZE
--------------------------|-----------
/Questbert.Core | 62.57 KiB
/Questbert.Service | 181.81 KiB
/Questbert.Service.Client | 50.26 KiB
/Questbert.Service.Game | 39.71 KiB
/Questbert.Test | 29.36 KiB
/Questbert.Tools | 53.91 KiB
/Questbert.Unity | 6.60 MiB
/Solution Items | 7.13 KiB
GIT: Get current revision
GIT: Current revision is a02551cdf64a9fe1149fa630f86c3dffb28f3445
postcheckoutstatus finished successfully.
Added new build target 'default-webgl'
Saved /BUILD_PATH/spottedzebra.questbert-unity.default-webgl/build.json
No Library cache found - ALL assets will be re-imported.
Using 2018+ DLL
+ Detecting Unity installation ...
+ Building with Unity installed at /UNITY_PATH/Unity/Unity-2018_2_14f1/Unity.app/Contents/MacOS/Unity
+ Installing UnityCloud build tools
Injecting custom Scripting Define Symbols 'UNITY_CLOUD_BUILD'
+ Switching Unity to target platform
+ Building Unity player (webgl)
! Unity player export failed!
! build of 'default-webgl' failed. exit
Publishing build 1 of spottedzebra/questbert-unity for target 'default-webgl'...
publishing finished successfully.
Zipping cache files from Library
tar: Library: Cannot stat: No such file or directory
tar: Error exit delayed from previous errors.
done.
Build step 'Execute shell' marked build as failure
postbuildstatus finished successfully.
Finished: FAILURE