welcome to the forums!
Is there a chance that the IP address that you assigned to the ESXi host is already in use by some other device?
Then you may randomly end up on the one or the other device.
Thank you all for your response.
I was being absolutly stupid and did forget that the DRAC of a system that has been configured at the same time unfortunatly was configured with the same IP as the ESXI - wondrously, after changing the DRAC IP, I was able to access the esxi...
Sorry for bothing you all