Can you please share which zimbra version are you using command :
zmcontrol -v
Also share if you are using the basic search or advanced search where you are if specifying the full email address of the sender or just the firstname?
Can you please once restart the proxy service and trace the errors in the logs here?
Logs location:/opt/zimbra/log/nginx.log & /opt/zimbra/log/nginx.access.log
both on proxy node.