This is a FFmpeg error. It looks like seekable output is a requirement of mp4 output. I would reduce your FFmpeg command down to just the essentials and test. The first thing I would do is remove the '-f mp4' option from the output. You are already running the input through libx264. There is no reason to stuff the output from libx264 into a MP4 container. The libx264 output stream will work just fine with librtmp and WMS.
Edit: Don't remove the '-f mp4' option but rather change it to '-f rtmp rtmp://localhost:1935/ffmpeg/myStream'