Hi, i have the same issue with the newest version of gillion, 3.7.3.1 beta. I am running the latest wordpress (5.5.1) fresh from the wordpress.org site and PHP 7.2.19 (stable).
update:
I switched to php 7.3.6 (stable) by now and am hoping that it will fix the issue. In fact, now i can actually install the theme and the backend of wordpress is still okay.
i refreshed the site with ctrl-f5 in firefox several times in between, so i am pretty sure it’s about the updated php version.