Datatype CLOB: lossy conversion when migrating from WE8ISO8859P1 to AL32UTF8

We have data of an application in a database of release 9.2.0.8 (characterset WE8ISO8859P1).

The data shall now be migrated into a database of release 11.2.0.3 (characterset AL32UTF8).

When executing the CSSCAN utiltity on the target database (9.2.0.8) I get the following result:

Table : MASTERCONTENT
Column: INFO
Type  : CLOB
Number of Exceptions         : 116      
Max Post Conversion Data Size: 1489532  

ROWID              Exception Type      Size Cell Data(first 30 bytes)    
------------------ ------------------ ----- ------------------------------
AAA4jBAAKAAACRcAAA lossy conversion 
AAA4jBAAKAAACRcAAB lossy conversion 
AAA4jBAAKAAAvNqAAA lossy conversion 
AAA4jBAAKAABg0nAAC lossy conversion 
AAA4jBAAKAABg0nAAE lossy conversion

...

From my understanding CLOB objects can store up to (4 gigabytes -1) * (the value of the CHUNK parameter of LOB storage)

of character data. Or is the problem that the characters of some rows can't get converted into AL32UTF8?

Currently I do not understand why "lossy conversion" will happen...

Any help will be appreciated!

Rgds

Jan