Hi @Dereje_Haddis,
this kind of errors are usually related to the SSL configuration in the server side. Android apps are a bit stricter than web apps when it comes to validate the SSL certificates. I was checking your instance url in online SSL checkers (such as SSL Geocerts Checker or SSL Shopper Checker and both of them say that the certificate is valid but the certificate chain is not complete.
This is something that must be modified in the server configuration (usually Nginx or Apache). Using the fullchain certificate should solve the issue. There was a similar issue some time ago regarding a Nginx server, maybe it could help you (API call threw SSLException).
Please let me know if it doesn’t solve the issue