If you are able to telnet to port 1521 and you get connected (you don't mention what happens when you telnet), then it seems unlikely that you're facing a firewall issue. I would start with some SQL*Net client tracing to see what your client is attempting to do during the connect. You can enable that with TRACE_LEVEL_CLIENT=16 in your sqlnet.ora on the client system. Then reproduce the error again and review the resulting trace file (especially at the end) and see what the client was trying to do just before the error happened. There is information in the Oracle documentation for Oracle Net (or Net8 or SQL*Net, depending on what version you look at) about performing tracing along with other parameters you may wish to set.
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