There are some forums on Internet with an explanation of the cause of this error [1], to summarize the error is generated because the version of OpenSSL installed in the server is too old and was not compiled with support for certificates signed with a specified encryption, in most cases “sha256WithRSAEncryption”. OpenSSL is being used by CURL to execute HTTP requests with HTTPs-based websites so that is why you are seeing that message in this plugin, I will offer you two solutions:
Easiest Solution
Go to the general settings page of the plugin and change the value of the option named “API request with SSL” to “Stop peer’s cert verification”, this will force the plugin to execute Curl with the SSL validations disabled.
Best Solution
Contact your hosting provider and request the upgrade of the version of OpenSSL installed in the server where your website is being hosted, tell them to install a version compiled with support for certificates signed with “sha256WithRSAEncryption” or a similar encryption algorithm.
[1] https://www.google.com/search?q=%22asn1+encoding+routines%3AASN1_verify%22