On the 18th of January 2018 mailgun updated their SSL certificates.
This can be seen by running the following curl command in terminal:
curl -vvv https://api.mailgun.net/
* SSL connection using TLS1.2 / ECDHE_RSA_AES_128_GCM_SHA256
* server certificate verification OK
* server certificate status verification SKIPPED
* common name: *.mailgun.net (matched)
* server certificate expiration date OK
* server certificate activation date OK
* certificate public key: RSA
* certificate version: #3
* subject: C=US,ST=California,L=San Francisco,O=MAILGUN TECHNOLOGIES,
INC,OU=MAILGUN TECHNOLOGIES\, INC,CN=*.mailgun.net
* start date: Thu, 18 Jan 2018 00:00:00 GMT
* expire date: Wed, 18 Mar 2020 12:00:00 GMT
* issuer: C=US,O=DigiCert Inc,OU=www.digicert.com
CN=Thawte TLS RSA CA G1
* compression: NULL
As you can see the certificate have been valid from Thu, 18 Jan 2018
around the same time your issue started.
What does this mean for you. It probably means you machines certificates files are out of date. Updating these files will allow you to valid the new mailgun ssl certificate.