Problem:
The customer couldn't launch serial sessions on any of his ACS48's from Dsview 3.7.2.91
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Environment:
Dsview3.7.2.91 with 5 ACS48's where the ACS48's are on a different subnet.
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Resolution:

The customer was using IP tables on his Dsview3 server. So after disabling the IP tables and using the proxy mode in Dsview3 everything worked fine.