Great, thanks for the help.
I believe that it has not expired, as the validity of this BT.CN issuer is from 30/06/2031.
Also, we installed a new VPS, to check this possibility too.
The behavior of the new installation is the same.
In other words, all Servers from different locations and installation dates have the same problem.
However, a few hours after I posted here, the certificate started working, with the browser only displaying the self-signed alert, but loading it normally.
That is, the situation is now normal.
We noticed that the certificate worked fine for FTPS using TLS 1.2 during this period, which is strange.
That is, just for the https login on the panel it did not work.
I repeat, once again, that we tested in various locations, installation dates and accesses, to eliminate the possibility that only one certificate has expired or the PC/browser that originated the access has some problem or firewall.
Thank you for your attention, as always!