In Vault Pro 2012 we also had the issue with the error 155 during assign item and the solution was mentioned in this article Error 155 When Trying To Assign Item on Vault Pro 2012.
However if you have this error "Error 155 Illegal Null Parameter" in Vault Pro 2013 the solution mentioned here (checkout file, rebuild all in Inventor, check back in) usually doesn´t help.
You will need to reinstall the Vault Server 2013.
Before this you should be sure that you have a valid backup of your Vault.
Steps to reinstall the Vault Server:
1. Go to Control Panel
2. Go to Programs and Features
3. Go to Autodesk Vault Professional 2013 (Server) and choose "Uninstall/Change"
4. On the Setup Pick "Repair or Reinstall"
5. Select Reinstall and start the operation
If this was successful the issue with error 155 should be resolved.
Update 28th February: the reinstall option will only recreate and re register the dlls in the Vault Server program folder and won´t do anything with the database!
As consequence you would have to perform this operation on all sites when using Workgroup replication.