Others have reported this problem. https://www.toadworld.com/products/toad-for-oracle/toad_for_oracle_beta_program/f/86/t/14882
It seems to happen when characters put into the MODULE or ACTION column are incompatible with the database character set via DBMS_APPLICATION. It can also happen with the OSUSER column.
Toad 12.12 works around the problem, so if you update your Toad the problem should go away.