Sorry Chris Fox I meant 6.25.0. You'll need to install the openjdk devel package (that provides jstack), then once the server hangs, execute something like:
I've been seeing a lot of these "Attempt to redefine loader" errors shortly after puppetserver has started and is taking some agent traffic. The errors go away after being thrown 1-3 times. stacktrace-attempt_to_redefine_loader.txt
FYI this same issue (redefine loader) occurs on server 7.x when the 7.14 agent is instealled: SERVER-3160 which seems caused by this change in the 7.14 agent: PUP-11373.