The issue occurred because the Supplier information was replaced in the Accounting System. To fix this issue:
Copy Apvend.dbf and Apvend.cdx.
Then go to C:\UBSACC2015\COMPANYDATAFOLDER.
Paste both folders.
Log in to UBS I&B.
Go to einvoicing, Configuration, Supplier.
Search for the Supplier.
Set the Send Default method again.