So I was one one the people who commented. I will explain my reasons for the reason I voted to delete your answer.
I also use Shrew Soft VPN client on win 10 and get this error message randomly.
I read this sentence as an auditor and see somebody with a problem. I read this problem, and put myself in the position as somebody with the same problem, and I am left wondering the reason you are telling you get this error message. When I do have a problem, and a Superuser answer helps me, I just want the solution.
Usually rebooting my PC makes it disappear and let me connect correctly again...
I read this sentence as an auditor one half of a two sentence comment to an answer. I put myself back in that position, with the same problem as before, and I am wondering what about the times it does not work.
for some time... until it comes back :-( It seems to be a software issue in the Shrew client.
I don't see this statement as being helpful. So the problem is the client, so what is the solution to the problem, rebooting doesn't solve the problem since it comes back after awhile.