Crashplan stopped backing up due to corrupt cache

Today I noticed that Crashplan running on my NAS was no longer backing up any files and that the backup set was 0 Mb and only 2 files (which should have been a few 100k files and > 350Gb). Rescanning the fileset didn’t help, neither did removing and adding the folders again.

After a little digging I noticed in the logs entries like (log message was a log longer but I only included the relevant part of it):

com.code42.exception.DebugException: BSM:: SET-1: Exception adding source file...skipping - fileStat=FileStat[/volume1/photo, exists = true, fileType = 1, length = 0, lastModified = 1419895925000, lastAccess = 1425690867000, created = 1419895925000], com.code42.backup.manifest.FileManifest$CorruptFileManifestException: CORRUPT FMF ENTRY FixedPortion[entryPosition = 186768232, fileId = 00000000000000000000000000000000, parentFileId = 00000000000000000000000000000000, fileType = 0, version = Version[timestamp = 0, sourceLastModified = 0, sourceLength = 0, sourceChecksum = null, fileType = 0]

Googling for this message did not render any result unfortunately and this part of the Crashplan system is rather obscure (nothing to debug, messages are limited. The only thing I could think of to try to resolve it was to drop the cache Crashplan maintains (in the cache subdirectory of the Crashplan installation). It turns out that this was sufficient as after a restart the cache was rebuild and the the scan resulted in the expected number of filed.

The steps I performed were:

1. Stop the Crashplan engine
2. remove all files in the Crashplan cache/ subdirectory
3. Start Crashplan
4. Enforce a rescan of the fileset in [Settings] –> [Backup] –> Verify Selection [Now]

Since I had removed the folders from the backup set I feared that I had to upload all data again to my external backup targets, but Crashplan was smart enough not to need that.