POP3 Proxy hasn't changed in 7.306. Does your log contain lines beginning with "Prefetch:" ? There may be a problem with the servers ip addresses imported from 7.2xx.

Please try following steps:
1. In Webadmin go to POP3->Advanced.
2. Click Edit for "Imported #1".
3. Add a new DNS group with the same hostname entered in your mailclient.
4. Remove all IP addresses from the list.
5. Save settings

Then repeat these steps for your second server.