1. Create a mp mission using 'new editor' in non dev version and save it and export it to mpmissions. Play it and edit it as much as you like with no problems.
2. Update to the dev version. Select edit the mission in mp. Make a change and save & export to mp missions, if it doesn't allow you to do this keep trying till it does.
3. Now try playing and re-editing in mp ... it may make the mission act erratically and when you try to re edit it you can ... but when you come to save you get the save failed warning.