As mentioned in this post, saving projects in OneDrive, Dropbox and the like is asking for problems.

Could you please check with a new project living outside of your OneDrive folder?

Thanks for your answer, Jesús. Apparently this is not the problem. As you can see in my screen captures, the files that do appear without failure every time I open SDL, are also stored in OneDrive.
Anyway I just gave it a try and created a proyect locally (C:\Users\Roland2020\Documents\Studio 2021\Projects) and the problem persists. The created project is not reflected in the project file listing.

Thanks for your answer, Jesús. Apparently this is not the problem. As you can see in my screen captures, the files that do appear without failure every time I open SDL, are also stored in OneDrive.
Anyway I just gave it a try and created a proyect locally (C:\Users\Roland2020\Documents\Studio 2021\Projects) and the problem persists. The created project is not reflected in the project file listing.

OK, then I'd carry on the following actions:

- Reset window layout (View menu > Reset Window Layout). Does it work?

- Otherwise, close Trados Studio, rename the PROJECTS.XML file (living in your C:\Users\[UserName]\Documents\Studio 2021\Projects\ folder) to something such as PROJECTS_OLD.XML. Open Trados Studio, it will re-create automatically the PROJECTS.XML file. Does it work?

That was it, Jesús. Second solution seems to work. Old projectos.xml file was not updated since december 4th for some reason.

Many thanks!

Probably the XML file was corrupted and Trados couldn't read it.

Just realised that I hadn't included the link in my first post here about storing projects in any cloud service folder. here it is:

https://community.sdl.com/product-groups/translationproductivity/f/studio/31421/using-trados-on-dropbox-on-different-computers

¡Me alegro de que el problema esté resuelto!