Sometimes the DBAppender failed to log Exceptions, so I checked the status of the StatusManager and found the folloging problem:
java.sql.BatchUpdateException: ORA-12899: Wert zu groß für Spalte "SUBSYSTEMTEST"."LOGGING_EVENT_EXCEPTION"."TRACE_LINE" (aktuell: 692, maximal: 254)

Possible fix:
After reading the code of DBAppender I subclassed it and changed the code of the method
void updateExceptionStatement(PreparedStatement exceptionStatement, String txt, short i, long eventId)
so that the text would be shortened accordingly.
This seems to be the most consistent workaround, as many other columns are also only 254 bytes wide.

I'd much rather have a wider column and not loose information, though. (This would be consistent with the changes to LOGGING_EVENT_EXCEPTION as proposed in LBCLASSIC-170).