Diodon password manager no longer stores history in a file (~/.local/share/diodon/storage.xml) as this approach caused limitations with history size
To work around this, Diodon now uses Zeitgeist on the backend. Zeitgeist has a privacy feature where you can set to not record history. So if this is set, the clipboard manager wont work anymore.
There is no entry in these settings to allow just the clipboard, so if you want Diodon to work, you need to turn on "Record file and application usage" and then select "Documents" from the include list.
Once set you need to restart Diodon, and then clipboard history should be working.