I just upgraded my desktop to ArcGIS 10.3 and none of my Oracle database connections work. I can't even figure out how to create new connections. SQL Server connections work fine. The error message is "Failed to connect to the specified server. Do you want to continue? UnderlyingDBMS error[ORA-01455: converting column overflows integer datatype. No extended error.]"
Oracle Database is 11.2.0.3 and existent connections were configured using "sde:oracle11g:<<instance>>" in the instance text box.
Any help is appreciated...
Apparently the problem was related to the Oracle client version. Altought I have both 64 and 32 Oracle 11g clients in my box (having the 32-bit client first in my path), I experimented the same problem until I download the Instant Client from ESRI and set it in my system's path as the first option. Hope this help others with the same problem...