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The following causes have been reported with firewalls by SonicWall;
Large packets: See Thin Client Registration fails Troubleshooting: Device Registration Fails with Error Message ": Unexpected end of input stream".
SonicWall DPI-SSL replaces the UMS certificate: If SonicWall DPI-SSL is enabled, it functions as intermediate CA and sends its own certificate to the devices instead of the original UMS certificate. As a consequence, the devices refuse to register because they would only accept the original UMS certificate.
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