Interesting that the exception states it can't find the constructor instead of not being able to find the class, but unfortunately it is not showing the code within TopLink Essentials that throws the exception. Can you get the stack trace to the internal exception? Also, looks like you are using an older build of Toplink essentials, can you try with one of the last versions of TLE 2 or 2.1? I don't know if any issues were resolved, but it can't hurt.
Unfortunately, from the description of EclipseLink bug 288015 that you have linked, you will still get the same issue in TopLink as they will use the same identity strategy for SQLServer. You might want to try a workaround - potentially, you could override the NativeSequence's buildSelectQuery() method to return ValueReadQuery("SELECT IDENT_CURRENT('tableName')", and then use a session customizer to add these via the getLogin().addSequence(sequence) method.
Best Regards,
Chris
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