Critical Error During Check in

[Guest]

We upgraded to 3.0.6 yesterday. Some users are getting the following error while trying to check in files:

Critical Error! Error in the application.
at VaultServiceBase.VaultRepository.GetReferenceCopiesByPath(HybridDictionary hdNewRefParents, String strAbsolutePath, Boolean bRefCopyLastObjectWithAllFolders, VaultFSObjects& fsoList, Int32& nPinCount)
at VaultServiceAPILib.VaultTransactionContainer.GetReferenceCopiesByPath(String strAbsolutePath, VaultFSObjects& fsoList, Int32& nPinCount)
at VaultServiceAPILib.VaultTransaction.PreCheckCheckIn(Int32 nCurrStatCode, VaultRequestCheckIn vrci, String strFileToken, VaultResponseCheckIn vRespCheckIn, VaultTransactionContainer txContainer)
at VaultServiceAPILib.VaultServiceAPI.BeginTx(HttpApplicationState theApp, VaultLoginInfo vliLoggedInUser, Int32 nTxUserID, Int32 nRepID, DateTime dtBeginTx, String strComment, VaultRequestItem[]& requests, String& strTxID, VaultIntTx& vit)
at VaultService.VaultService.DoBeginTx(HttpApplicationState has, HttpSessionState hss, VaultLoginInfo vli, String strCallerLogEvt, Int32 nTxUserID, Int32 nRepID, DateTime dtBeginTx, String strComment, VaultRequestItem[]& requests, String& strTxID)

Again, this only happens to some users. I, for example, am able to check out and check in files from the same repository in which they are having trouble.