We are seeing an issue with using the SAP Business One Solution Packager to package a copy of the database and include the Boyum UDO tables. The issue can also happen after a DB upgrade.
The issue is that you will be unable to create new UDO entries in Universal Functions/Item Placement tool/Function Buttons/other UDO windows as you will get the error "The entry already exists in the following tables". This will happen even if you use codes that do not already exist.
We have received a copy of a customer database with the issue and we are able to replicate the issue but it is an SAP bug.
If you create a default form on the UDO object and it gives the same error.
As this is a UDO object we use SAP functionality for saving/updating it so we cannot give an answer to why it is not working.
Our recommendation is to contact SAP support and say that you have a UDO default form that you cannot update (Attached are screenshots of the default form based on the Universal function table) and ask for a fix.
See this video for more details and examples on how to make the default UDO: