I had an error in Transform stating the target source does not follow the naming convention.
After investigation, it was caused by the fact there was a leading space in the field "Add Row Source Override" in the vertical of "target source"-page of Map. (see attached)
We changed this value.
Seems this field is required so is filled with a space by default on initialization.
It should be more user-friendly however filling a space by a validation rule on submitting the page.
This would prevent the error in Transform. Also to note is that Autogen didn't give an error, so he accepts the name.
It should be checked if more required fields have this behaviour.