Recently had a customer with a similar error and a restart fixed it (since as I mentioned earlier the DB state is typically kept consistent). In your case I can't explain / don't have any idea what may have caused your current state. It can theoretically be fixed manually via SQL, but the data structures are generally quite complex to understand, so I am quite hesitant to suggest / provide any SQL for this as it is very likely it will be used incorrectly and end up breaking your system even more...
Essentially, you will need to remove the alf_prop_* entries referencing that non-existing property value, and cascade that cleanup to all entries referencing those removed alf_prop_* entries (and so on, and so on...)
