Thank you for your post.
Does this occur with other files or just a specific file? If this only occurs with a specific file, have you tried a Recover on the file? If so, were there any error messages in the Recover.log file?
As I'm currently only working with this file I cannot tell if this happens with other files as well - I will keep an eye on it. Anyways it seems the problem is gone since I unchecked "Add newly defined fields to current layout" in the preferences.