Too many open files - what kind of beast is this?

Question

Java

Added this code:

try (
  FileOutputStream fout = new FileOutputStream(filepath);
  ObjectOutputStream oos = new ObjectOutputStream(fout)
) {
  oos.writeObject(item);
}

Called frequently (5 times per second), after a day the application crashes with Too many open files.
Before this code worked for months...
What am I doing wrong? Thanks in advance!