Unable to connect to Oracle 11g server remotely using SQL developer ORA-12520
A couple of things can trigger this error. My guess is this the first thing below.
Max Process Limit Reached
You may have have exceeded the maximum number of processes. (see here and here.)
This query will show you the current utilization, max utilization, initial allocation, and limit for Processes:  select * from v$resource_limit where resource_name in ('processes','sessions');. You will need to run the query from the server, since you can't get in remotely. If Max Utilization is at or near your limit value, this may be the problem.
You can see what the process limit is by logging into Oracle as sysdba from the server. show parameter processes will display something that says processes and has an integer value. You can extend this value by issuing this command: alter system set processes=XXX scope=spfile; set XXX to a higher value. Oracle recommends 200 for a &quot;small&quot; system and 400 for a &quot;medium&quot; system. You will have to shutdown and restart Oracle to enact the change.
Oracle bug
Oracle mentions a Bug, bug number 13831723, in Oracle 11.1. Your information seems to indicate you're running 11.2, so this shouldn't apply. But if it does, then you'll need to install a patch from Oracle. Log into Oracle support and search for Oracle DOC ID 1566704.1. They say it is fixed in 11.1.1.7.0 and up.