Solved: Puppet Enterprise Console cannot read classes from Puppet Server
The environments and classes on my Puppet Enterprise Console had become unavailable. On the Puppet server, in the puppetserver.log file I found an error that occurred every time I was tried to reload the classes from the Console. The essence of the error was: "Input to add-path-to-file-entry does not match schema".
After a while I found out that the Puppet agent (1.3.6-1) on the Puppet server machine did not match the version of Puppet Server (version 2.4.0 from PE 2016.2.1). All it took to resolve the problem was to update puppet-agent to the version correct from the PE 2016.2.1 release, that is, puppet-agent-1.5.3-1.