I have found the problem. Everything was OK with the installation. After I cleared the browser cache in Chrome, it worked. Thanks for your support!
I have attached the entire log from the first login on the Raspi including the OMV installation.
The installation of OMV starts in line 487.
From line 6946 the output of "omv-sysinfo" starts. In the section "Network Connection" (from line 7091) there are many error messages.
I tried reinstalling OMV on a Raspberry 4 several times today. I always have the problem that the web GUI is not accessible (Google Chrome error message: "ERR_CONNECTION_REFUSED").
I have always used the latest "Raspberry Pi OS Lite" (Release 4 March 2021) and the installation script for OMV.
In my last try I really did everything exactly step by step as described in the instructions "Adden-B-Installing_OMV5_on_an R-PI.pdf" (incl. SHA check, H2testw, ...).
The "sudo service nginx status" via SSH/Putty looks OK at first glance "Active: active (running)".
The /var/log/nginx error logs are empty.
The /var/log/openmediafault folder is also empty (is this OK?).