This time I captured the error message.

That is why I do not dare close the file and Fusion in fear of losing my data.
I've looked into my Fusion log. I do not know much about what I look for, but here are a few excerpts that looks suspicious:
It seems that I'm about to save two files at the same time, which I do not believe I am doing.
:6 Error: Not Found [404] Response:[object ArrayBuffer]
20150129T080340 I *I* *** Validate check fixed 4 command state[s]. LastCmds=DashboardModeCloseCommand,SelectCommand, Changes=DashboardModeOpenCommand(E1), DashboardModeOpenCommand(V1), DashboardModeCloseCommand(E0), DashboardModeCloseCommand(V0)
20150129T080856 I Auto-save is temporarily disabled for document: BattBox all-fit v134 - 20150129T080856
20150129T080956 I Started auto-saving document: /Users/boldsen/Library/Application Support/Autodesk/Autodesk Fusion 360/TQMPME87NF9N/PLM360Cache/a360proue29cba76.autodeskplm360.net/1/BattBox all-fit.471783a2-c6bb-48f5-9ad0-3011e628457a.f3d - 20150129T080956
20150129T081000 I Finished auto-saving document: BattBox all-fit v134 - 20150129T081000
20150129T081500 I Started auto-saving document: /Users/boldsen/Library/Application Support/Autodesk/Autodesk Fusion 360/TQMPME87NF9N/PLM360Cache/a360proue29cba76.autodeskplm360.net/1/BattBox all-fit.471783a2-c6bb-48f5-9ad0-3011e628457a.f3d - 20150129T081500
20150129T081503 I Finished auto-saving document: BattBox all-fit v134 - 20150129T081503
20150129T082003 I Auto-save is temporarily disabled for document: BattBox all-fit v134 - 20150129T082003
20150129T082103 I Auto-save is temporarily disabled for document: BattBox all-fit v134 -
20150129T090433 I Queued file /Users/boldsen/Library/Application Support/Autodesk/Autodesk Fusion 360/TQMPME87NF9N/PLM360Cache/a360proue29cba76.autodeskplm360.net/UploadQueue/01030424-b450-476b-960b-dcb2976aa36f.AutoSave in queue 2. Original File: BattBox all-fit.471783a2-c6bb-48f5-9ad0-3011e628457a.f3d
20150129T090434 D CreateNewDocTask::onExecute
20150129T090434 D CreateNewDocTask executed: NewDocumentCommand
20150129T090435 I Number of files waiting to be uploaded, or in the process of getting uploaded: 1
20150129T090435 I Number of files waiting to be uploaded, or in the process of getting uploaded: 1
20150129T090435 I Number of independent files we can process at the same time: 1
20150129T090435 I Number of independent files we can process at the same time: 1
20150129T090435 I Number of Uploader threads currently: 2
20150129T090435 I Number of Uploader threads currently: 2
20150129T090435 I Upload speed of thread 0 was: 0
20150129T090435 I Upload speed of thread 0 was: 0
20150129T090440 I Number of files waiting to be uploaded, or in the process of getting uploaded: 1
20150129T090440 I Number of independent files we can process at the same time: 1
20150129T090440 I Number of Uploader threads currently: 2
20150129T090440 I Upload speed of thread 0 was: 0
20150129T090445 I Number of files waiting to be uploaded, or in the process of getting uploaded: 1
20150129T090445 I Number of independent files we can process at the same time: 1
20150129T090445 I Number of Uploader threads currently: 2
20150129T090445 I Upload speed of thread 0 was: 0
20150129T090448 I *I* *** Validate check fixed 4 command state[s]. LastCmds=DashboardModeOpenCommand,SelectCommand, Changes=DashboardModeOpenCommand(E0), DashboardModeOpenCommand(V0), DashboardModeCloseCommand(E1), DashboardModeCloseCommand(V1)
20150129T090450 I Number of files waiting to be uploaded, or in the process of getting uploaded: 1
20150129T090450 I Number of independent files we can process at the same time: 1
20150129T090450 I Number of Uploader threads currently: 2
20150129T090450 I Upload speed of thread 0 was: 0
20150129T090455 I Number of files waiting to be uploaded, or in the process of getting uploaded: 1
The size of the file was 11213897
20150129T090515 E
PLMException. PLM to N2 transfer wasn't complete for file /Users/boldsen/Library/Application Support/Autodesk/Autodesk Fusion 360/TQMPME87NF9N/PLM360Cache/a360proue29cba76.autodeskplm360.net/1/BattBox all-fit.471783a2-c6bb-48f5-9ad0-3011e628457a.f3d with item 2084 in function updateAttachment on the 1 attempt and 0 requeue.