I installed the template. Uptate to acutal Joomla-Version (3.8.x) not possible? 3.6.5 is possible, but if I try to update to 3.8.x (via backend update check) it breaks an the backend is gone).

Also I have a problem with php. I run php 7.1 on the web server (7.2 is also possible).
I get the messages: Warning: A non-numeric value encountered in /www/htdocs/w01887f8/andrea-kraus-neukamm.de/plugins/system/helix3/core/helix3.php on line 490

Installed with quick installation.

Need help, thanks