619 just means you could not connect, ANYTHING in the path could be causing this.
A (simplified) process:
VMC Dashboard->Windows OS->Drivers->PC hardware->Data card->radio tower->transmission line->Radio Controller->transmission line->authentication servers (some interconnected by transmission lines)
If any component, in the path, gives problems, the end-to-end connection can't be established and Windows will return a generic "I could not connect' error, the dreaded 619.
The above is just the physical path, there are also a lot of configuration parameters as well:
1) Is the SIM provisioned for data.
2) If on pre-paid, do you have credit.
3) Are you using the right APN. (I've made this mistake a few times

)
4) Are the DNS settings, etc. correct.
Once you've been issued an IP, there still might be problems, but they'll typically give other errors such as HTTP, routing, DNS, firewall, etc. errors.
So, 619 is often more a pain than anything else, as it does not give any indication of where the problem might be. The only way to narrow it down is to look for common reports, thus the importance to get accurate location information. The call desk will then use the above path to try and find the cause.
1) If only one report comes from a specific area, it can very well be on the PC side.
The fact that so often a reboot fixes the error points to a problem on the PC side.
My own setup never gives 'local' 619's, I probably got two 619's in the last month and both times there was a actual network problem.
So it worth it spending time to get your PC side right. I know it's not always simple.
2) If multiple problems are reported form a specific area, it could be a tower or transmission line that's down.
3) If it comes from all over the show, probably a back-end system, etc.
When you guys reports problems here, the first thing I do is look at your location, but unfortunately not everyone completes the field.
Also, please report the problem. So often, you guys suffer in silence and no one knows there's a problem!