Camtasia Studio 8 was recently "upgraded" to Camtasia 9.
In case "version 10" was merely a typo and you have never had a problem "with latest version" that suggests something changed between saving the file and reloading.
I have had weird issues with Windows 10 where something changes and THEN Microsoft tells me of a "pending" update. Checking for and handling the updates fixes the issues. If that is not the problem you might consider reinstalling Camtasia to restore any lost, or changed, CODECs. (There may be a better method than reinstalling...) If that fails, consider contacting support.