One of the files used by DSA got corrupted.
Re-installing DSA on the same computer might not help but installing a new DSA on another computer has proven to resolve the problem.
For instructions how to install a new DSA please see SOL35842:
If new DSA works fine then this new DSA could be used in future or the old one could be re-installed to resolve the problem.
In some situations (when re-installing DSA is not desired or does not help) this approach can be used:
1. Delete all existing (and failed) migration sessions. Before doing this please read the following Quest KB articles - SOL45104 and SOL45091:
If you have directory synchronization configured and working then deleting migration sessions is not a concern.
2. Restart the migration server.
3. Launch Migration Manager and confirm directory synchronization is stopped.
4. Close Migration Manager.
5. Rename the three .xml files (jobs.xml, activation.xml, resolver.xml) on the DSA machine, files can be found under %Program Files%\Quest Software\Migration Manager\DSA\CONFIGS\
6. Launch Migration Manager, create a new migration session and migration should work now, error should disappear.
Note: this procedure will require a full resynchronization.