It works.
I uninstalled ffdshow, version 20050328
and installed 20050303, I think that has caused the trouble.
Now I have my xvid clips with subs in a mp4 container. Great.
One question:
I can choose srt or ttxt files as format to mux in mp4.
Which way is smarter, in order to produce a 100% conform playable mp4 (for a stand alone device with mp4 support)?