Alessia,
Your problem is not Windows Operating System related, and I would not upgrade to 5.9 since there are limitations (SSF record editor will not run), and 5.85 is the perfect pair for TerraSync 5.86. TerraSync 5.9 is also buggy, with a memory leak that hampers effective data collection using a DDF.
So 3 things to check.
Refresh or UPDATE List in the Diff Wizard. Do this once/month.
COR stations at the top of the list will be the closest station, but these may either be down and not reporting, or you did not wait a full 24 hours before the data commensurate with your rover data is posted.
Check the box before Diff. correction to ensure your text box of the process will reveal your overlap of rover data with the base. Are you getting 100% of your positions covered in time? Once you find the station that does give you 100% or nearly that, proceed thru the steps
Finally, does the reporting tool then provide you with the % of data that is corrected. I think you said that was the case.
Open COR file and using the View by Time window, check your features are corrected.
Get back to me.