Sometimes issues appear on the Enterprise Vault server post installation that seem very odd. Perhaps they're problems with conversion or processing particular messages. Sometimes the answer lies in relaying down the Enterprise Vault binaries, sometimes the answer re-registering a particular DLL, or set of DLL's. This final step is done as part of the re-installation of binaries, but a quicker method exists.
There is a file called FileReRegister.bat, which is quite useful. If you open it and take a look you'll see lots of entries in it, registering DLL files like this:
It's worth a try to open an elevated command prompt, navigate to the Enterprise Vault program folder, and executing the batch file. Have you ever used it? Did running it solve the underlying issue you were experiencing? Let me know in the comments below.