Critical error while processing request: /

I'm trying to use recoll-webui in apache2 with WSGI/CGI but I get the error "<h1>Critical error while processing request: /</h1>" in my apache error log and in the browser.

System:

ubuntu 16.04.6 (headless)
Recoll 1.28.5 + Xapian 1.2.22 (from ppa:recoll-backports/recoll-1.15-on)
Apache/2.4.18 (Ubuntu)

I have installed ubuntu packages libapache2-mod-wsgi-py3 and python3-recoll

Apache virtual server config (redacted):

<VirtualHost *:80>
        ServerName recoll.myserver.com
        WSGIDaemonProcess recoll user=recoll group=recoll threads=1 processes=5 display-name=%{GROUP} python-path=/var/lib/recoll-webui
        WSGIScriptAlias / /var/lib/recoll-webui/webui-wsgi.py
        WSGIProcessGroup recoll
        <Location />
                AuthType Basic
                AuthUserFile *********
                AuthName "*********"
                <Limit GET>
                    Require valid-user
                </Limit>
        </Location>
</VirtualHost>

This worked (with minor bugs) with recoll-webui from https://github.com/koniu/recoll-webui