Unable to connect to pluggable database in Oracle 12c
Is the pluggable database (PDB) online?

Connect to the container database where you attached the PDB. Issue this SQL command: show pdbs; This should list all of your Pluggable Databases (including the seed PDB) and what state they are in.

You can start the DevDB database via this command: alter pluggable database DEVDB open; 

If that works and the database opens and comes online, you can tell Oracle to "remember" this state via:  alter pluggable database DEVDB save state; 

If, however, the database does not come online via the alter...open command, check select * from PDB_PLUG_IN_VIOLATIONS; for possible problems.

From your edits, I see that the PDB is up and available. Try connecting to the DEVDB from another computer? 

When you connect to the container, you can then switch your context to the PDB via alter session set container = DEVDB;

In my Oracle 12 environments, I do not have the PDBs listed explicitly in the server's TNSNames file (just the container DB). On client PCs, I then list the PDB in the TNSNAMES file.

On the server, I use the ORACLE_SID environment variable and do not use the @service, so: Set ORACLE_SID=DEVDB and then sqlplus user/pwd 
(But that is in a Windows environment.)