This is one particular bug that bugs the most. Hard to find and you just need to keep asking your users to restart and one should be very lucky to get connected to SonicWALL VPN.
The issue description is mostly like, your SonicWALL client just disappears after some time when you try to connect. And it acts very rude you cannot guess when it connects when it does not. And SonicWALL client does not show any error message it just disappears and also in the CMS or Device user sessions it just shows connected with kb's of data movement.
So, after a lot of struggle and research and support from the SonicWALL support team we got to know that there are a lot of issues with respect to V3 attributes. If you are still using V3 attributes in the configuration of your SonicWALL profiles, the get rid of it to resolve the issue.