I am very thankful for all of your support, as you know when issues with clients' sites arise, and the solution is not apparent, it's challenging to find an answer. Knowledgeable contributions are vital to sort out the errors and find a solution as the ones provided by all of you.
In the end, it was an issue with the hosting provider server (reseller Linux shared server https://www.resellerclub.com/reseller-hosting);
I moved my website to another hosting provider (siteground.com). Their customer services were fantastic and got it back online on Sunday afternoon, meeting what we offer to our client.
The first transfer did not work and was still showing errors, but we try a second transfer, and it worked perfectly, and since then, the website has not had any issues (knock on wood).