ORA-12502 TNS:listener received no CONNECT_DATA from client on Oracle 11g RAC environment.

If you get "TNS-12502: TNS:listener received no CONNECT_DATA from client" errors in your listener.log or in your SCAN listener logs. Or some client connection gives "ORA - 12502 TNS:listener received no CONNECT_DATA from client" errors (check client sqlnet.log for these). Then you know there is some kind of connection problem going on.

There is troubleshooting guide for this kind of problems in MOS (My Oracle Support) (this advice to set sqlnet listener tracing on if errors are coming rarely.This way you can get more info about problematic connection (where it is coming etc.) But if these errors are coming frequently then it is not wise to set listener tracing on because you will get huge log file. If you get this error from multiple clients then your application probably provide invalid connection string.):
MOS document: [ID 453505.1]

If you have RAC environment you can also read following MOS (My Oracle Support) document (this help you to check that your remote and local listener settings are correct):
MOS document:[ID 453293.1]

If everything else looks to be ok then problem is probably in firewall/network settings. Check that you have opened both SCAN IPs and VIPs from client to server. If you open only SCAN IPs then you can get these errors because RAC uses internally also VIPs.