After Update Mysql 8.0.13 to 8.0.15 MySql Server Fails with error code = 1265
I found the solution myself. 
On the old mysql 8.0.13 server I corrected the field SQL_MODE of one of the procedures, what happened to be too long.

SELECT * FROM INFORMATION_SCHEMA.ROUTINES;


I found some (old) stored procedures I did not need anymore, and the entries in field SQL_MODE of one of that stored procedures were too long.
After deleting that stored procedure, starting the new 8.0.15 server and "mysql_upgrade" everything is fine, the server can now start without any errors.