Thanks for reporting this, Thomas. [Issue](https://github.com/GluuFederation/oxTrust/issues/309) is created an we are working on this.
If you need it to be resolved asap, our current opinion about the cause of that is as next: when first attribute is deleted, it's also purged from OpenDJ's schema, but user entries that had some values assigned to it keep those. When you try to delete next attribute, it also tries to update user entries (can't say atm for what reason though), and fails because according to schema such attribute doesn't exist anymore.
Long story short, after removing all values assigned to attributes you can't delete (plus the one you deleted) from all user entries inside Gluu with tool like `ldapmodify`, you should be able to delete the rest of your attributes too.