When you configure eZ Platform to handle front SiteAccesses with Map\Host matcher and admin SiteAccess with Map\URI you will encounter infinite spinner in Page Builder Preview, when previewing in different hos (front SiteAccess), then matched for admin.
Steps to reproduce
1. Clean installation of eZ Platform 2.5.3
2. In ezplatform.yml:
3. Go to: http://site.local/admin. Open "Home" Landing Page in PageBuilder in site2 SiteAccess.
Page preview is displayed as intended.