As of January 15, 2020 Sonatype, the hosts of the Maven Central repository will no longer be permitting insecure connections to their repository via http. More information on the why´s can be found here.
So if you´re working on a Grails application that uses the mavenCentral() repo, and suddenly on a rebuild or refresh of dependencies you find MavenCentral depenedencies are failing to resolve filling your log with a whole stack of the following errors:
SERVER ERROR: HTTPS Required <URL of POM/jar>
Then the solution is to modify your buildConfig.groovy and replace mavenCentral() with the secure HTTPS explicit URL. You can go ahead and replace all the default insecure repositories as such.
grailsPlugins() mavenCentral() grailsCentral()
mavenRepo "https://repo.grails.org/grails/plugins/" mavenRepo "https://repo1.maven.org/maven2/" mavenRepo "https://grails.org/plugins/"
Once updated, you should find your dependencies pull down without failure once again.