Without the newline iCal complains in the logs:

11-06-01 16:11:06 iCal[12955] Unexpected EOF, returning last token as fallback

radicale/ical.py

@@ -52,7 +52,7 @@ def serialize(headers=(), items=()):
     for part in (headers, items):
         if part:
             lines.append("\n".join(item.text for item in part))
-    lines.append("END:VCALENDAR")
+    lines.append("END:VCALENDAR\n")
     return "\n".join(lines)