Server inaccessible after upgrade from 7.5 to 7.6

Hi.

I got 2 VPS, yesterday I upgraded one, from OpenBSD 7.5 to OpenBSD 7.6 (amd64),
today I wanted to upgrade the remaining one, after "sysupgrade -nk" and "reboot",

I cannot login to the system anymore (I manage only via SSH), Putty says: "Remote side unexpectedly closed network connection",
after entering the password of my root user. I tried ssh in the Windows terminal, same thing happens.

ssh server is running, login: prompt arrives. And I see that my ssh client disconnects after issuing the correct password.

If the password is wrong, it attempts to ask for it again.

I got no physical or console access to the server.

Any idea would be much appreciated.