Error messages of this sort are often that a process was not able to open a particular file and write to it. Typically (but not always) it has to do with the workstation or network environment, such as having Opportunistic Locking enabled or an overzealous anti-virus checker holding onto files, or a data file seen as open by the system even when it is not.

I doubt there is anything in particular that you did wrong. Perhaps the next time you want to perform Day End, make a 'related data' backup beforehand. If the error occurs again, there is a 'good' backup of the data, and something that Softrak (or your Adagio business partner) could use to help trace down the problem cause.