cannot deserialize arraylist - but why?

this is the error message i get:

Caused by: com.google.gwt.user.client.rpc.SerializationException: java.util.ArrayList/4159755760

at com.google.gwt.user.client.rpc.impl.SerializerBase.getTypeHandler(SerializerBase.java:153)

at com.google.gwt.user.client.rpc.impl.SerializerBase.instantiate(SerializerBase.java:114)

at com.google.gwt.user.client.rpc.impl.ClientSerializationStreamReader.deserialize(ClientSerializationStreamReader.java:111)

at com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamReader.readObject(AbstractSerializationStreamReader.java:119)

at com.webfair.gwt.components.genericforms.shared.ButtonRow_FieldSerializer.deserialize(ButtonRow_FieldSerializer.java:18)

at com.webfair.gwt.components.genericforms.shared.ButtonRow_FieldSerializer.deserial(ButtonRow_FieldSerializer.java:36)

at com.google.gwt.user.client.rpc.impl.SerializerBase.deserialize(SerializerBase.java:95)

how to find out what exactly is causing the error? the arraylist itself obviously cannot be the problem, i've sent a lot of them back and forth.

everything the list contains implements isserializable...

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