The Enterprise Vault Client-Driven PST Migration process converts folders which contain backslashes ('\') into separate Folder-SubFolder paths.
For example a folder named 'First\Second\Third' would be enumerated by the Client-Driven PST Migration process to have the following folder structure:
Any items contained in the folder 'First\Second\Third' would be placed in the 'Third' sub-folder.
Have users pro-actively rename folders that include '\' characters with a different character. Identify PST files that have folder names with at least one '\' character, then utilize an alternate Enterprise Vault PST Migration process.
This issue has been addressed in the following release:
Enterprise Vault 9.0.4 Cumulative Hotfix 5 Release
Enterprise Vault 9.0.5 - Release Details
Enterprise Vault 10.0.3 Cumulative Hotfix 2 Release
Enterprise Vault 10.0.4 - Release Details