Thanks for your answer.
In fact I had to wait 6 hours more….
Really strange but then the restarts of the apache server stopped and I could access the web site. This also allowed to control the behavior of wordfence.
Before I started wordfence on another web site I changed
<IfModule mpm_winnt_module>
ThreadStackSize 8388608
</IfModule>
Maybe I should have done that before…
The second wordfence instance worked just fine from the very beginning!