I've found that
Error #2038 is related to the size of files to upload. We have a limit of 10 Mb and when trying to upload files larger than that, this error occurs on Apple computers.
The strange thing is that on Windows computers we get a regular alert explaining about the file size, and on Apple computers we get this weird technical error message. Why is that?
Anyway, the original main problem persists:
When using Safari on an Apple computer, the files don't get uploaded. Not even when they are way smaller than 10 Mb.
This is specific to Safari. When using Firefox on Apple, everything is fine.
What causes this weird Safari issue?
Regards, Marja