indexing - rpc timeout on insert with no error in logs

I have a rpc_timeout on a insert of one single row (the cluster is with 3 nodes, and can handle more than 10000 insert / min on another table)

Here is the table :

CREATE TABLE test_table (
agent text,
run_id text,
process_id text,
datetime timestamp,
tracking_time timestamp,
email text,
ip text,
event_id uuid,
event_name text,
message_id text,
source text,
url text,
test_table text,
PRIMARY KEY ((process_id, event_name), event_id));
CREATE INDEX test_table_process_id ON test_table (process_id);

and here is the insert :

BEGIN BATCH
INSERT INTO test_table (message_id, run_id, event_id, ip, process_id, agent, datetime, event_name, url, test_table, email, tracking_time) VALUES ('exampleaaaaaaaaaaaaaaaaaaaaaaaaa', 'bar', 376d8e20-35ca-4615-8e9f-f0b5b4431981, 'None', 'test-dummy', 'None', '2014-08-31 17:20:24', 'hard_bounce', 'None', 'mandrill', '[email protected]', '2014-09-01T18:04:40');
APPLY BATCH;

I don't know if the timeout is due to the secondary index.

Nothing about any error in system.log