Stash Backup Client fails with »does not appear to be a valid home directory« for Stash 3.3.1

2014-10-01 08:13:35,370 INFO  [main] c.a.s.i.b.client.spring.SpringMain Initializing
2014-10-01 08:13:36,627 INFO  [main] c.a.s.i.b.client.RestStashService Using Stash 3.3.1
2014-10-01 08:13:36,696 INFO  [main] c.a.s.i.b.client.DefaultBackupClient Contacting Stash
2014-10-01 08:13:36,730 ERROR [main] c.a.s.i.backup.client.BackupMain A backup could not be created. Reason: The configured STASH_HOME, C:\Atlassian\Stash, does not appear to be a valid home directory. Creating a backup requires direct access to the home directory for the running instance to allow it to interact with the repositories.
com.atlassian.stash.internal.backup.client.BackupException: The configured STASH_HOME, C:\Atlassian\Stash, does not appear to be a valid home directory. Creating a backup requires direct access to the home directory for the running instance to allow it to interact with the repositories.
	at com.atlassian.stash.internal.backup.client.layout.DefaultStashHome.verify(DefaultStashHome.groovy:52) ~[stash-backup-core-1.4.0.jar:na]
	at com.atlassian.stash.internal.backup.client.DefaultBackupClient.backup(DefaultBackupClient.groovy:43) ~[stash-backup-client.jar:1.4.0]
	at com.atlassian.stash.internal.backup.client.spring.SpringMain.run(SpringMain.groovy:28) [stash-backup-core-1.4.0.jar:na]
	at com.atlassian.stash.internal.backup.client.spring.SpringMain.run(SpringMain.groovy) [stash-backup-core-1.4.0.jar:na]
	at com.atlassian.stash.internal.backup.client.BackupMain.main(BackupMain.groovy:38) [stash-backup-client.jar:1.4.0]
	... 27 frames suppressed