The process cannot access the file [filename] because it is being used by another process. (2019 Freelance+)

I've had this problem for ages with 2019. I have tried all the tips mentioned in all the threads I found on the subject, but nothing works. It only happens when I "create a new project", not if I "translate as a single document". I have 2017 running on the same machine and it never throws up this error. I usually reboot and reboot and reboot, and eventually it will start to work on 2019. However, for two days running I have been unable to start a job, as it is persistently giving this error message. Perhaps upgrading to 2021 will help? :)