Invalid Project File

I’m having a problem opening files. Projects that I’ve only been working on in the last few days are now suddenly refusing to open, with an “Invalid Project File” error message. On this particular project, I have one bakup file which can open, but is many hours out of date and essentially useless.

The only thing I can think of is that I load and save projects from and to an external harddrive, but this has been scanned and checked and is working fine.

Is there anything I can do? The filesize seems right. On a laptop, I get a different error message, stating that the project was created in a different version of Cubase and can’t be opened. As I have the same Cubase installed on both PCs I don’t know how that can be so.

Cubase Artist 8.5 on Windows 10.

I have just updated my Cubase version to 8.5.30. The error message has now changed to match the laptop, saying that the project was created with Cubase version 8.5.10 and cannot be opened. I’m beginning to lose hope, but also trust in this.