How to customize error message in Server Apps ?

There's some instructions on how to do that for older versions of FME Server, which at least should point you in the right direction: https://community.safe.com/s/article/customizing-data-download-and-job-submitter-servic

Keep in mind that that will change the messages coming from the services themselves, so they'll be applied accross your entire server, rather than just Server Apps.

Thanks you but my FME server is recent

It looks like the paths mentioned for the 2017 version are still correct (at least on an express installation of FME Server 2021)

Ok I'm going to try, even if I use "data streaming"