Please post the contents of the sqlnet.ora and sqlnet_ifile.ora from the database tier.
EBS R12 requires the use of Developer 10g.
Which exact version of EBS R12 ?
Please see (Client / Server Interoperability Support Matrix for Different Oracle Versions (Doc ID 207303.1)).
If you're on R12 and the database version is 11.2.0.x/12.1.0.x then you need to install a higher client version for this to be supported.
Instead of using Dev6i with R12 use 10g Developer -- Using the OracleAS 10.1.2 Forms and Reports Builders with Oracle Applications Release 12 (Doc ID 444248.1)