I recently noticed this error in /var/log/kern.log.1 :

Sep  2 16:55:59 simon-laptop kernel: [193370.279532] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-28]
Sep  2 16:55:59 simon-laptop kernel: [193370.279542] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-28]
Sep  2 16:55:59 simon-laptop kernel: [193370.279548] Error writing headers; rc = [-28]
Sep  2 16:56:03 simon-laptop kernel: [193373.918985] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-28]
Sep  2 16:56:03 simon-laptop kernel: [193373.918995] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-28]
Sep  2 16:56:03 simon-laptop kernel: [193373.919001] Error writing headers; rc = [-28]
Sep  2 16:56:07 simon-laptop kernel: [193378.222126] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-28]
Sep  2 16:56:07 simon-laptop kernel: [193378.222135] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-28]
Sep  2 16:56:07 simon-laptop kernel: [193378.222141] Error writing headers; rc = [-28]
Sep  2 16:56:22 simon-laptop kernel: [193393.160557] ecryptfs_encrypt_page: Error attempting to write lower page; rc = [-28]
Sep  2 16:56:22 simon-laptop kernel: [193393.160566] ecryptfs_write_end: Error encrypting page (upper index [0x0000000000000018])
Sep  2 16:56:22 simon-laptop kernel: [193393.160686] ecryptfs_encrypt_page: Error attempting to write lower page; rc = [-28]
Sep  2 16:56:22 simon-laptop kernel: [193393.160692] ecryptfs_write_end: Error encrypting page (upper index [0x0000000000000018])
Sep  2 16:56:31 simon-laptop kernel: [193402.293173] CIFS VFS: No response for cmd 50 mid 50075
Sep  2 16:56:31 simon-laptop kernel: [193402.312303] CIFS VFS: No response for cmd 50 mid 50081
Sep  2 16:56:35 simon-laptop kernel: [193405.440576] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-28]
Sep  2 16:56:35 simon-laptop kernel: [193405.440585] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-28]
Sep  2 16:56:35 simon-laptop kernel: [193405.440591] Error writing headers; rc = [-28]
Sep  2 16:56:40 simon-laptop kernel: [193410.478510] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-28]
Sep  2 16:56:40 simon-laptop kernel: [193410.478519] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-28]
Sep  2 16:56:40 simon-laptop kernel: [193410.478525] Error writing headers; rc = [-28]
Sep  2 16:56:40 simon-laptop kernel: [193410.478733] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-28]
Sep  2 16:56:40 simon-laptop kernel: [193410.478740] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-28]
Sep  2 16:56:40 simon-laptop kernel: [193410.478745] Error writing headers; rc = [-28]
Sep  2 16:56:42 simon-laptop kernel: [193413.325000] ecryptfs_encrypt_page: Error attempting to write lower page; rc = [-28]
Sep  2 16:56:42 simon-laptop kernel: [193413.325009] ecryptfs_write_end: Error encrypting page (upper index [0x00000000000000e1])
Sep  2 16:56:48 simon-laptop kernel: [193419.202905] ecryptfs_write_metadata_to_contents: Error attempting to writSep  2 17:14:17 simon-laptop kernel: [194467.438781] Valid eCryptfs headers not found in file header region or xattr region
Sep  2 17:14:17 simon-laptop kernel: [194467.438790] Either the lower file is not in a valid eCryptfs format, or the key could not be retrieved. Plaintext passthrough mode is not enabled; returning -EIO
Sep  2 17:19:39 simon-laptop kernel: [194789.437221] Valid eCryptfs headers not found in file header region or xattr region
Sep  2 17:19:39 simon-laptop kernel: [194789.437229] Either the lower file is not in a valid eCryptfs format, or the key could not be retrieved. Plaintext passthrough mode is not enabled; returning -EIO

And some of these :

Sep  3 09:40:38 simon-laptop kernel: [54047.132631] ecryptfs_decrypt_page: Error attempting to read lower page; rc = [-4]
Sep  3 09:40:38 simon-laptop kernel: [54047.132640] ecryptfs_readpage: Error decrypting page; rc = [-4]

Since then, I get a lot of those :

Sep  2 17:19:39 simon-laptop kernel: [194789.437221] Valid eCryptfs headers not found in file header region or xattr region
Sep  2 17:19:39 simon-laptop kernel: [194789.437229] Either the lower file is not in a valid eCryptfs format, or the key could not be retrieved. Plaintext passthrough mode is not enabled; returning -EIO

Those error occurred after I ran out of space in my home. I don't know if it worth mentioning but when this occurred I was running this kernel : Ubuntu 2.6.35-19.25~lucid1-generic 2.6.35.3. I am now back to the default Ubuntu kernel : Ubuntu 2.6.32-25.43-generic 2.6.32.21+drm33.7 and I still have the error.