The change from single user to multiple users does not change the program or any of the functions within the Sage 50 CDN program. Your problem may have arisen because of a change made to an entry from a prior, already reconciled, month. I try to find these before starting the actual reconciliation by following these steps. First choose the account and enter the statement end date and reconciliation date. Tab to the Statement end balance but do not enter it yet. The Statement Opening Balance shown should equal the opening balance on your bank statement as well as the unresolved amount at the bottom of the screen. If there is a difference then there were changes made to entries prior to the current month. Determine the amount of the difference and look for an entry(ies) with a date prior to the start of the period being reconciled which match the amount or total the amount. Verify that these were entries that had already been reconciled in a prior period and then check them off. Your unresolved should now total the same as the Opening Statement Balance. If this is the case you are ready to enter the Statement End Balance and carry on with your reconciliation.
This same situation could happen if a change was made to an entry in the current month and only one half of the reversing entry was automatically checked off. Before starting a reconciliation check that if there is a check mark against an entry there is also a check mark against the entry being reversed. Again before starting a reconciliation the unresolved should equal the Statement Opening Balance when the Statement Closing balance is zero. This was more of a problem in some of the older versions of the program but not as much of a problem in the current version.
Hope this helps

Clyde, thank you so much for the response. When I try to change the dates in the account reconciliation window I get an error message saying: "invalid date. the statement end date must be between Aug 1, 2015 and Dec 31, 2015. I can't go back to previous months. How would I do this? Thank you

You can't go back to look at prior reconciliations online, you should be printing the reconciliation report(s). I prefer the Summary Report with Outstanding Transaction Detail as this provides the detail required to reset the reconciliation for the beginning of the following month. Some people prefer the Detail Report which provides all transactions that have been cleared as well as the outstanding transactions.
If you are trying to look for changes that may have been made check the comments column on the Transaction tab when the Reconciliation module is open and the bank account is showing. A reversing entry will show as "Rev. of Jxxx. Corr. is Jxxx" This line should be checked along with the entry it reversed.
Any further sleuthing would have to be done in the General Ledger for the bank account but this does not show if an item has been reconciled or not.