Hi Alessandro
The attribute was probably previously enabled or there is a naming conflict with another attribute.
Get the attribute unique ID and then run the query on the PPM database:
(Replace c_custom_field with the field name)
select * from dwh_meta_columns where attribute_code ='c_custom_field'
Note the results - is this the same object? Does it say is_deleted=1
Then please let me know. You can attach the results in Excel format. We may have to open a Support case for that depending on the results.
Thank you -Nika