RelationList's selectionMethod in legacy could be configured as one of six different options:

1. Browse
2. Drop-down list
3. List with radio buttons
4. List with checkboxes
5. Multiple selection list
6. Template based, multi
7. Template based, single

(ref: https://github.com/ezsystems/ezpublish-legacy/blob/master/design/standard/templates/class/datatype/edit/ezobjectrelationlist.tpl#L10)

Nevertheless, new stack supports only two of them:

1. Browse
2. Dropdown

(ref: https://github.com/ezsystems/ezpublish-kernel-ee/blame/5.4/eZ/Publish/Core/FieldType/RelationList/Type.php#L32)

Even after migration from legacy, ContentType configuration stored in database still contains selectionMethod values from legacy. Both PlatformUI and AdminUI can't handle it properly what makes saving of ContentType impossible.