This is the second time I've had this problem in the last couple of weeks – it's not a problem I had had previously.
When I try to start a new multiple document project, when I click 'OK' (or whatever the button is) in the project configuration window, instead of listing the files as it usually does, CT goes straight into the editor, which is blank (resources opened, but a blank grid). When I then click on the orange 'Start' button I get the message 'No source file'.
The project is a mixed project with 2 docx and 1 odt file.
I had the same problem last week and found a workaround, but I don't recall why, whether there was anything special about that project or what that workaround was.
P.S: Forgot to mention, I'm still on the final Akua release.
So I've found the bug responsible for this problem:
In order to exclude a specific file from being imported by CT, I'd renamed it from filename.docx to filename.docx__
Then I noticed that the CT status bar was displaying filename.docx__ – so CT obviously doesn't filter the file types properly. Presumably if the file was called docxdemo.xyz, it would include that too.
If I rename it to filename.dxxocx, CT does now start the project correctly.
This needs to be fixed.
The 2019 Forerunner version skips the files ending with a non-letter character so you might use this feature to filter them out.