Error: ERROR 1047 (08S01) at line 6955: WSREP has not yet prepared node for application use
I had the same issue with gcloud percona instance running on 3 servers. 

After running mysql logs from the first instances i found out it was not able to reach to instance-2. I also ssh to instance-2 but i could not reach it. On tailing mysql logs in instance-3 i found out its also not able to reach instance-2. 

I restarted instance-2 and everything went back to normal. 
With percona mysql clustering when one cluster is down and dirty_write turned off all the write operations are blocked untill all the clusters are restored.