Error during migration

Hello community!

We just started a migration from BOE XI R2 to BI 4.1 using the upgrade management tool.

Everything works fine for 15000 objects except 350 WEBI.

The error is:

while trying to invoke the method com.crystaldecisions.sdk.occa.infostore.ISecurityRight.setGranted(bool) of a null object loaded from field com.businessobjects.sdk.plugin.desktop.common.internal.migration.MigrationUnifiedSecurityFactory$2$1$1$1.val$rightToWrap of an object loaded from local variable 'this'

Do you have any idea what could be the problem ?

Thanks in advance for your help.