I’ve been experiencing the same issues as @mirjam. It was fixed temporarily by moving our VPN hubs from Germany to Bahrain/Australia as you can read in this post: Flaky and unstable VPN connection with RUTX10 .
However, the past few days Bahrain/Australia are starting to show the same problems. I’d really like Teltonika to solve this issue, as their VPN hubs are currently just not stable enough to use in production environments.
Today I had to tell our customer that we could not look into issues at a system, because VPN was not working, ouch…