Dalamud crashes on launch with "The file exists" error

[Jessidhia]

System.IO.IOException: The file exists.

   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.Path.InternalGetTempFileName(Boolean checkHost)
   at XIVLauncher.Dalamud.DalamudLauncher.Download(String addonPath, Boolean staging) in D:\Sapphire\FFXIVQuickLauncher\XIVLauncher\Dalamud\DalamudLauncher.cs:line 195
   at XIVLauncher.Dalamud.DalamudLauncher.Run(DirectoryInfo gamePath, ClientLanguage language, Process gameProcess, Boolean doDownloads) in D:\Sapphire\FFXIVQuickLauncher\XIVLauncher\Dalamud\DalamudLauncher.cs:line 103
   at XIVLauncher.Dalamud.DalamudLauncher.DoWork(Object state) in D:\Sapphire\FFXIVQuickLauncher\XIVLauncher\Dalamud\DalamudLauncher.cs:line 48
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart(Object obj)
Version: 5.2.0.0
Git Hash: 7f46f54
Context: Unhandled
OS: Microsoft Windows NT 6.2.9200.0
64bit? True
DX11? True
Addons Enabled? True
Auto Login Enabled? True
Language: English
LauncherLanguage: English
Game path: ...\steamapps\common\FINAL FANTASY XIV Online

Addons: []

It looks like the code tries to guard against this issue with an .Exists() check but it seems GetTempFileName() just throws an exception instead. Now, why would GetTempFileName() throw an error...

[goaaats]

Make sure there's space on your hard drive, clear out your temp folder, then try again - see if it still errors out

[Jessidhia]

There was, uh, an impressive amount of mostly 0KB files named tmp[0-9A-F]{1,4}.tmp, going all the way to FFFF; however, deleting only (all of) them did not help with the error... I kept deleting more things from %temp% and then eventually it started working, not sure what exactly I deleted that helped.

[Jessidhia]

After updating plugins and then closing the game, a tmpE572.tmp file was left in the %temp% dir; the contents are a PE file (well, dll):

It seems that there are some temp file leaks around...

[goaaats]

I doubt this is a problem with XIVLauncher, but if it comes up again we'll have a look.

[Jessidhia]

The error has returned again, this time when using the patch installer; at any rate, using the official launcher to finish patching and then using quick launcher again got the same error. There's some file leak somewhere, in either the launcher or Dalamud...