Reloading the page... shows the page. closing and restarting seamonkey re show "this connection is untrusted" and so on.
The trick was kaspersky suite used by company; it act as a "man of middle". When updating version, the certificate was add only to firefox, chrome, edge and ie.
Others browsers where ignored

The ITY guy first tell me: "Use Chrome, it works!" which is silly: the source problem is not Seamonkey (but IT service can say: "you have a "solution", next

To resolve the problem, i found this:
https://community.kaspersky.com/kaspers ... ssing-1161
Bingo, import Kaspersky certificate into seamonkey and now all is OK. I still use Seamonkey

Hope this can help.