If you receive an error similar to the below when trying to run a download session...
The sync state generation is not found; generation=17;[highest=31][31][29][30]
...this means that the program can no longer use New messages only option to download the email. The reason for this error is that the email server has lost a track of when the last download occurred and so can no longer download new messages only.
If you are using the New messages only option, make sure you set the download frequency to at-least once an hour or once a day, depending on how many emails you receive.
PRO Server NOTE: If you are using the PRO Server and have a service installed when you get this error, uninstall the service first before trying these steps.
Workaround
To overcome this, you need to switch back to All mail and then run a download session, then switch to New messages only.
The workaround usually requires you to change the Search In criteria (either in Search tab or the Global Filters tab) from "New messages only" to "All mail". Then add a date filter (date greater than) to a date of todays date either in the Global Filters tab or the Search tab or in rules to avoid downloading all mail all over again. Then run a Connect and Download session as you normally would. Then switch back to New messages only.
IMPORTANT: Make sure that you run the connect and download sessions at-least once a day when the "New messages only" option is selected to avoid getting a stale synchronization state.
Alternate and more robust fix
Rather than using New messages only another option is to set this to All mail and then use Global Filters to move the email to a different folder (eg. Processed) on the email server.
For this, you need to create a rule or edit the existing rule and add the Move email action to a new mailbox folder (eg. Processed). This way when the program completes processing an email, those emails are then moved to a processed folder and the program will not re-process the same email.