You are currently working on UAT 

object reference set to an instance of an object

When I create a new project or translate as single document from certain txlf files, I receive the above message and cannot proceed with translation.

I can successfully create a new project or translate as single document from some txlf files but I cannot with other txlf files.

In either case, I can open all txlf files with Wordfast Pro 5 without a problem.

Will you advise me what to do?

I am using SDL Trados Studio 2019 SR2 - 15.2.7.2849 which I believe is the latest version.