The most likely cause of this error is that the data was cleared and depending on your settings this may include all paid invoices. The only way to recover them would be to restore from a back up prior to the data clearing. Hopefully this was done very recently as all input since the backup was made will have to be re-entered.

It started happening after I updated to 2023 so I think I will go back to 2022 until the fixes are made. Only a couple of days of data.

Thanks

During the installation process and when you roll forward to a new year you are asked if you wish to delete records. If this is accidentally checked and you continue ahead there is no going back except to restore a backup. This is not likely to be caused by an error/bug in the 2023.o version of Sage 50C as I have not seen anyone else commenting about this as an issue of conversion. I will know better when I start to convert my client files this weekend. 

Thanks Alwyn, as it was not our year end I don't think this is the issue. I have restored from a back up and will work with the old version for now.