You should be able to customize Single Event Template the way you want by following this guide: https://docs.myeventon.com/documentations/customize-single-events-page/
Unfortunately, we unable to provide support for customization requests. You can send us a customization job request by creating a new ticket in here and selecting Customization Request as the category.
Please comment margin: 0 .25% .5%; in EventON > Styles:
.ajde_evcal_calendar.boxy .eventon_list_event {
width: 49.5%;
/*margin: 0 .25% .5%;*/
}

No issues on this page:
https://www.waldviertelakademie.at/veranstaltungen/

Could you send us a screenshot?
calendar view and extended event list, don’t work
Could you elaborate? Apart from tile_count, what also doesn’t work?
You can login with tester/pass and then go to this page: http://dev.myeventon.com/wp-admin/post.php?post=49662&action=edit:
Please send us wp-admin access to your site as a private message for us to further assist your situation.
Unfortunately, there is no option to change this color, so please add this CSS code to EventON > Styles (If you can’t see any change, EventON > turn off Write styles to header > save settings > turn on Write styles to header):
.ajde_evcal_calendar.cev #evcal_list .eventon_list_event a span, .ajde_evcal_calendar.cev #evcal_list .eventon_list_event a em, #evcal_widget .ajde_evcal_calendar.cev .eventon_events_list .eventon_list_event .evcal_desc .evcal_desc_info em { color: black !important; }

Hello,
The problem is in your server. Please check our test server: http://dev2.myeventon.com/21208-2/ > go to APR1 27 – MAY 3, 2020.
I doubt object-fit: cover; object-position: center; will work in this case since we aren’t using an image but rather a span HTML tag with background-image.
Please add this CSS code to EventON > Styles (If you can’t see any change, EventON > turn off Write styles to header > save settings > turn on Write styles to header):
.ajde_evcal_calendar.boxy.boxstyle0 #evcal_list .eventon_list_event .desc_trig, .ajde_evcal_calendar.boxy.boxstyle0 #evcal_list p.desc_trig_outter { padding: 0 !important; }

I have attached images as examples of the events being displayed on the day they are taking place
You can also try downgrading to EventON 2.7.3 to see if it helps: https://www.dropbox.com/s/4kr060y45fa9cmm/eventON_2.7.3.zip?dl=0