The SNMP Cisco ASA VPN Connections sensor queries a couple of different OIDs, but on some ASA versions, one of them is not returned by the device, Active Email Sessions.
This device template (download link) creates an SNMP Custom Advanced sensor which includes all the other OIDs, which are still returned:
- Active Internet Protocol Security (IPsec) sessions
- Active LAN-to-LAN (L2L) sessions
- Active LB sessions
- Active sessions in total
- Active switched virtual circuit (SVC) sessions
- Active users
- Groups with active users
The template is used by the auto-discovery. Please put it into the respective folder, the default path is C:\Program Files (x86)\PRTG Network Monitor\devicetemplates
Then, open the device's "Settings" tab, set "Device Identification and Auto-Discovery" to "Auto-discovery with specific device templates" and select "SNMP Cisco ASA VPN Connections (Custom)", then save to start the auto-discovery.