I found changing connectivity from mobile to wifi and back often gave me the same error. 400, push token error.
After playing with firewalls and all the network settings a lot, I found disabling the PUSH functionality immediately gave me a correct connection! But it warns correctly that you would not receive calls that way. The error is in this function, which I purchased for lowest energy consumption.
So, it wasn't firewalls or anything, there is an error in this push module in the background...
At least I have connectivity again, but far removed from ideal...
Then I tried the recommendation for Huawei phone, and entered App Info, Battery usage, Optimizing battery usage, allow to be managed ON, and then get it OFF the managed list again. It was unmanaged, but I added it specifically to the list, and took it off again...
It was without PUSH when I did this. (checkmark in notification area, ZoiPer running without push enabled.)
I activated push again, the ZoiPer service was restarted, Checkmark disappears during...
And then it CAME BACK! Yesss!! This seems to work!
I run this on a Samsung Note 8, latest update...