We have used your blogpost: https://premium.wpmudev.org/blog/move-multisite-new-domain/ to migrate a single wordpress installation to multisite.
Everything goes well and no error has been found. Till we try to save a category in a blogpost.
We found that something must be wrong with the database because the sidebar to add category, posttypes etc. is not saving. (we can add a futureimage though)
Check the error here: http://heldendaad.nu/dberror.jpg
Can you help us figurate this out?