Network error FRM-92102 and F5

A R12.1.3 EBS site has one concurrent server and two web/forms servers which are monitored by F5 for load balance. If I started Apache services on two web hosts, forms got network error very quick. But if I started services just on one host, this problem would not happen. The error is

FRM-92102: A network error has occurred. The Forms Client has attempted to reestablish its
connection to the server 5 times(s) without success. Please check the network connection and try again later.

The Java Console log showed " oracle.forms.net.ConnectionException: 500 " error.

I believed this was something in the network, not in the EBS apps config. I worked with F5 admin on this and they found Persistence profile in F5 setups was null for this site somehow. After they set the value to two hours, this problem does not happen again in this site.

I had similar problem in another R12 site before that, when forms got FRM-92102 errors after user opened forms for a few minutes. That problem was caused by a wrong timeout value for Persistence profile in F5.