dbua is failing during 12c upgrade: "you do not have enough tablespace free space or disk space to complete the upgrade."

During a 12c upgrade from 11.2, using dbua, I've received the error message above, even that all the tablespace had enough disk space and no space shortage in any file system either.
 The dbua trace file mentioned that the error was related to UNDOTBS tablespace.

The solution was to make the UNDOTBS datafile exensible, and the dbua went on :-)

SQL>alter database datafile '/mydb/ora_data02/undo_MYDB_01.dbf' autoextend on;

The issue here was that the dbua was failing, even that all the logs indicated that the space was OK.