There is a conflict between Domain mapping and the Thrive Architect page builder from thrivethemes.com
The Thrive editor works just fine on subsites that are not domain mapped. However, when mapped subsites are accessed at their original unmapped URL, the Thrive editor does not fully load (loader just spins), but there are no relevant errors in browser console. When those same subsites are accessed at their mapped URLs, the Thrive editor loads and works just fine though.
This appears to be due to recent changes in Domain Mapping as things were working fine with version 126.96.36.199 prior to updating to 188.8.131.52