Thank you for your answer. We checked it again and the folder is really empty.

However, we noticed that the error only appeared when we changed the source language. This is probably the reason why it happened only with certain customers (with a source language other than German).

We are testing the settings of the project template now. After adding e.g Italian>ES, FR, DE, EN it worked for these language pairs.

But how does this setting work? Creating a project worked for all projects with German as a source language even though not all language combinations were listed there. And the error message is not related to this.

Aaaaaha, so the steps are actually different from what you stated originally ;-)
This nicely demonstrates how important every little piece of information can be...

Then this might indeed point to a bug in Studio...

I can imagine that when you start the wizard with a set of language combinations, Studio creates its folder structure in the project folder (which is indeed empty)... and then, when you change the source language, Studio wants to "start over" and create its folder structure again... but fails because it didn't clean up the previously created structure, i.e. the project folder is not empty anymore. And it does the cleanup - unfortunately too thorough, so that even the previously existing "4_CAT" folder is deleted - only after clicking out the error message.

You can easily verify if my thoughts are correct by observing the content of the "4_CAT" folder e.g. in Windows Explorer in parallel with performing the steps.

We had the 4_CAT folder in Explorer open the whole time and it stays empty.

When testing this we sometimes encountered a slightly different phenomenon:

Instead of changing the path to "Translation_test\Translation_test" Trados now changed it automatically to the path of the project that was created before (and naturally that folder is not empty). Of course we can change that path again to the desired one.

But sometimes the other error occurs.

We had the 4_CAT folder in Explorer open the whole time and it stays empty.

When testing this we sometimes encountered a slightly different phenomenon:

Instead of changing the path to "Translation_test\Translation_test" Trados now changed it automatically to the path of the project that was created before (and naturally that folder is not empty). Of course we can change that path again to the desired one.

But sometimes the other error occurs.

Then you need to follow up with SDL on this, as this sounds like a bug in Studio.

Meanwhile you can check out my STraSAK, as what you are doing looks like an ideal candidate for automation instead of all that annoying manual clicking...
Though, certain level of technical knowledge would be required...