I got it to work, turns out the ESXi firewall rule permitting access thorugh port 443 hast to be reconfigured from allowing a bigger network (that vRLCM was placed in!") to "allow any". Also re-running the pre-check a few times sometimes showed that the communication is still not there or the result was, that the vIDM hostname was not resolveable in DNS (which is not true). So the installer seems to be a bit janky.