Gentle advice, from “How to ask questions…”
“It’s not useful to [say] what you think is causing your problem. …make sure you’re telling them the raw symptoms of what goes wrong, rather than your interpretations and theories.”
Following this principle, the content in your second post is what you probably should have started out with. The thread aspect was the wrong avenue to explore.
Keep in mind that the order of transformer execution and maps is undefined–they can be executed in any order. Have you stepped into the map step? You should be able to see which specific transformer is throwing the exception.
The class cast exception is probably the best thing to chase down. Is there a field or two, on either source or target side, that is not a string? Watch the pipeline carefully at runtime as it will show you the run-time type–which may differ from what design-time may show. E.g. a string var holding a date string may actually be a Date object at run-time (just an example–not sure what types you’re working with).