I found something for you. See below the quote from the sonic site knowledgeportal. It talks about a configuration that cause subsequent connections to be blocked.
It seems to me that you have a Sonic Wall issue and should try to post on their site or the tek-tips sonic site. Other than configuration issues, you will also need to look at the rom revision of the
Here is some info that might help you:
"SonicOS Standard (only if ‘Apply NAT and Firewall Rules’ is enabled) -- Log into the SonicWALL’s Management
GUI. Go to the ‘Firewall > Access Rules’ page. For both of the ‘Key Exchange (IKE”)’ rules, click on the
‘Configure’ icon to the right and click on the ‘Advanced’ tab. From there, adjust the ‘TCP Connection Inactivity
Timeout (minutes)’ field from the default of ‘5’ to ‘60’. When done, click on the ‘OK’ button to save and activate the
changes. NOTE: Do not adjust the ‘Default Connection Timeout (minutes)’ entry field found on the ‘Firewall >
Advanced’ page, as this applies to every connection through the SonicWALL, and may cause the connection
cache to fill up and prevent subsequent connections."