@epsilonadmin I checked the browser's console and there are no errors.
The following are some ajax requests and relative responses where you can see some progress (n_pending from 8 to 6) but I started a rebuild at 10am and it's still running at 20pm:

[["vr","status",{"n_inindex":1201,"n_actual":1193,"n_pending":8,"nw_act":0,"nw_total":0,"tsd":1626285118,"is_cached":true,"is_pause":"0","enabled":1,"index_ready":0,"autoreindex":1,"est_time":"--:--:--","ts":1626285269,"is_inx_outdated":0,"is_inx_notrun":0}],["vr","postdata",[]],["vr","code",0]]

[["vr","status",{"n_inindex":1201,"n_actual":1195,"n_pending":6,"nw_act":0,"nw_total":0,"tsd":1626285429,"is_cached":true,"is_pause":"0","enabled":1,"index_ready":0,"autoreindex":1,"est_time":"--:--:--","ts":1626285588,"is_inx_outdated":0,"is_inx_notrun":0}],["vr","postdata",[]],["vr","code",0]]