“Save and continue Editing” messes up what I write

Hi,

whenever I am writing a post and hit “Save and continue Editing” to preview what I have written, WordPress messes up my text:
It produces tags which shouldn’t be there (for example it closes tags although they were already closed thereby producing invalid HTML code) and it inserts spaces where they don’t belong.

That’s very annoying, what is this all about?

Thanks,
Matthias