Everything happens 51 minutes too late

I don’t think so.
As you wrote, your server does not have a RTC. As I understood, the issue you are seeing is not noticed on systems which do have an RTC. So it is more like an issue outside of openHAB.
A possible solution could be to somehow delay your openHAB start until your server/OS did synchronize time with an ntp server, but then you have to rely on an internet connection.