Delphi 10.0 :: Chunked File Upload with IOS App leads to Server Exception EConvertError

The app I work on uploads image files in chunks (sent as TStringstream) with a final TMultipartformdata using THTTPClient..
This system works 100% on Android but on iOS the server raises exceptions like this

Exception EConvertError: Invalid URL-encoded char (%Yπ) at position 69

Tried anything with encoding but nothing helps.
The app is developed with Delphi 10.0 Seattle Update 1,
the server uses a dll developed with Delphi 2007.