✔ 最佳答案
Both are grammatically correct, but each needs to be used according to the context.
"All is well " is used for describing a situation,
E.g.:
(1) "I haven't seen you for a long time, I hope all is well with you !".
(2) There was a big storm last night, but all is well now.
"All are well" is used for describing more than one person, or animal,
E.g.:
(1) During the big storm last night, the horses in the barn were terrified, but all are well now.
(2) My whole family fell sick last night after eating octopus soup. We took some medicine and all are well now.