Google App Engine Deploy Fails: "Unable to update"

[Mike via StackOverflow]

I have been deploying my app no trouble, then all of a sudden it fails. I am not sure what I have done to cause this. The error log is telling me that I need to perform a rollback. When I do that it says it succeeded. Then when I try to redeploy, I still get the same error!

If I try to deploy to a different version I still get the same error message.

The error log says:

Unable to update:
com.google.appengine.tools.admin.HttpIoException: Error posting to URL: https://appengine.google.com/api/appversion/create?app_id=codeavengers&version=alpha&
409 Conflict
Another transaction by user mrwwalmsley is already in progress for app: s~codeavengers, version: alpha. That user can undo the transaction with "appcfg rollback".

    at com.google.appengine.tools.admin.AbstractServerConnection.send1(AbstractServerConnection.java:293)
    at com.google.appengine.tools.admin.AbstractServerConnection.send(AbstractServerConnection.java:253)
    at com.google.appengine.tools.admin.AbstractServerConnection.post(AbstractServerConnection.java:232)
    at com.google.appengine.tools.admin.AppVersionUpload.send(AppVersionUpload.java:912)
    at com.google.appengine.tools.admin.AppVersionUpload.beginTransaction(AppVersionUpload.java:587)
    at com.google.appengine.tools.admin.AppVersionUpload.doUpload(AppVersionUpload.java:143)
    at com.google.appengine.tools.admin.AppAdminImpl.doUpdate(AppAdminImpl.java:419)
    at com.google.appengine.tools.admin.AppAdminImpl.update(AppAdminImpl.java:55)
    at com.google.appengine.tools.admin.AppCfg$UpdateAction.execute(AppCfg.java:1326)
    at com.google.appengine.tools.admin.AppCfg.executeAction(AppCfg.java:327)
    at com.google.appengine.tools.admin.AppCfg.<init>(AppCfg.java:210)
    at com.google.appengine.tools.admin.AppCfg.<init>(AppCfg.java:121)
    at com.google.appengine.tools.admin.AppCfg.main(AppCfg.java:117)
com.google.appengine.tools.admin.AdminException: Unable to update app: Error posting to URL: https://appengine.google.com/api/appversion/create?app_id=codeavengers&version=alpha&
409 Conflict
Another transaction by user mrwwalmsley is already in progress for app: s~codeavengers, version: alpha. That user can undo the transaction with "appcfg rollback".

    at com.google.appengine.tools.admin.AppAdminImpl.doUpdate(AppAdminImpl.java:425)
    at com.google.appengine.tools.admin.AppAdminImpl.update(AppAdminImpl.java:55)
    at com.google.appengine.tools.admin.AppCfg$UpdateAction.execute(AppCfg.java:1326)
    at com.google.appengine.tools.admin.AppCfg.executeAction(AppCfg.java:327)
    at com.google.appengine.tools.admin.AppCfg.<init>(AppCfg.java:210)
    at com.google.appengine.tools.admin.AppCfg.<init>(AppCfg.java:121)
    at com.google.appengine.tools.admin.AppCfg.main(AppCfg.java:117)
Caused by: com.google.appengine.tools.admin.HttpIoException: Error posting to URL: https://appengine.google.com/api/appversion/create?app_id=codeavengers&version=alpha&
409 Conflict
Another transaction by user mrwwalmsley is already in progress for app: s~codeavengers, version: alpha. That user can undo the transaction with "appcfg rollback".

    at com.google.appengine.tools.admin.AbstractServerConnection.send1(AbstractServerConnection.java:293)
    at com.google.appengine.tools.admin.AbstractServerConnection.send(AbstractServerConnection.java:253)
    at com.google.appengine.tools.admin.AbstractServerConnection.post(AbstractServerConnection.java:232)
    at com.google.appengine.tools.admin.AppVersionUpload.send(AppVersionUpload.java:912)
    at com.google.appengine.tools.admin.AppVersionUpload.beginTransaction(AppVersionUpload.java:587)
    at com.google.appengine.tools.admin.AppVersionUpload.doUpload(AppVersionUpload.java:143)
    at com.google.appengine.tools.admin.AppAdminImpl.doUpdate(AppAdminImpl.java:419)
    ... 6 more

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