Addendum: after removing the offending file, I get the laconic error message:

 (...and Community takes me to my original question above! ;-D)

Any ideas greatly appreciated!

Oh, and it would be awfully nice to have some kind of status indication about the progress of the conversion process, awfully nice indeed!

I got this too. Haven't the faintest idea why. All my files are available locally. The project works fine but can't be converted to the cloud.

The error message is 'incorrect file extension when generating sdxlifff files - file extension must be .sdlxliff.

What can I do?

Hi Mark Lewis , what version of Trados Studio is installed on your computer? You can check this information in Help -> About Trados Studio. The issue may have been caused by the fact that you do not have the latest version installed.

Alex J.

Hi Alex. I have version Trados Studio 2021 SR2 - 16.2.12.9338. Although why would this be a version problem?
Converting a local lproject to the cloud is standard isn't it? At least from whatever version it was introduced ...

I just wanted to make sure that you are running the latest update in order to avoid any possible Studio previous releases issues. Yes, a local project conversion to a Cloud project is standard but also depending on what license are you using. Can you please tell me what Studio license are you using? According to the Studio license limitations - Trados Studio Starter https://www.trados.com/products/trados-studio/editions-comparison.html , the Cloud projects can only be used and not able to save it to Cloud. 
Can you also send me the Trados Studio log file, but before you do that can you try once again to Save the project to Cloud so the error message stack trace will be logged there? Studio log files can be found here: C:\Users\[UserName]\AppData\Roaming\SDL\SDL Trados Studio\Studio16\logs

Alex J.

Hi. I was finally able to set up a project on SDL online!

The problem was in the project setup, field mapping options. I had selected something other than the default. When I selected default (trial and error between setting up the translation engine and memory etc.). Thanks for responding.

Hi - we are currently enhancing notifications in Studio 2022 so that in future, when you create a project yourself, you should get a notification in Studio that a translation task has been assigned to you once all processing has completed. So - a bit like an "automatic refresh" to let you know that processing is done and you can continue working on the project. This might already be available for the saving local projects to cloud flow, but I am not 100% certain. When you use the Create New Cloud Project wizard, it's available there and we'll bring it to more of these mixed local/cloud flows as well over time. Thanks, Daniel