Ich habe doch bereits geschrieben, dass ich mir das Backend angesehen habe und dass wir die vielen Dinge, die Sie noch machen müssen hier nicht supporten. Es hat nichts mit dem Template zu tun. Bitte lesen Sie meinen Thread dazu noch einmal durch.
Wir schließen den Thread dann auch in Kürze.

Thanks Dirk, but I did follow your advice, and I have installed a backup of the website in a separate folder. Then we remove the previous template since you mentioned that might cause the problem. We also uninstalled many components, plugins, and modules. (Basically, we only kept the ones we really needed.) Unfortunately, we still get the error: Error: Call to undefined method Astroid\Document::astroidCustomCSS(): Undefined class constant 'MSG_EMERGENCY'

Any additional advice?

In addition to my earlier message from today, I randomly installed another template using a different platform than yours, and it immediately worked. So it looks like the problems are caused by the template and/or platform and not by any of the plugins or components. I like the design of your template, but if it causes so much trouble installing it, I might want a refund of my purchase and that I move on to a different template and platform. Thanks. 

Leider nein, das müsste man detailliert untersuchen aber wie bereits mitgeteilt, ist das kein Thema für unseren Support.

Ok, then I want a refund of my purchase. You can refund it through my PayPal account, which was also used for the original payment. Thanks in advance.

For refund please mail us your Transaction ID
www.joomlaplates.de/kontakt.html