You are moving from WE8MSWIN1252 to AL32UTF8 (Unicode)

Your affected columns on target database are too small and can't handle the growth-in-bytes or some Unicode characters.

In your example above MYFIELD column is as big as 263 bytes while column size is just 255

You might want to check Metalink Note:119119.1

A workaround -if permitted by application, is to pre-create affected tables in Target database giving offending columns a larger size. In this case don't forget to use skip=y during Import.