Narrowed it down to what it does not seem to like through some trial and error. It's if I try to add the tag "Dynatrace Managed" is when it bombs. Not sure why.
After going into edit mode about 3 different times, it at long last took it.
Hello @Larry R.,
Thank you for noticing this and informing us! We are aware of this issue (unfortunately, it also includes the creation of Product Ideas and Articles and there is no clear pattern when and why this happens) and already submitted a bug to our forum engine provider and are in contact with them about this. They are working on resolving this bug, but we don't have a timeline for a fix yet.
Sorry for all the inconvenience, we hope that it won't take long.
From the end user perspective it looks like as some session stickyness goes wrong - your request is probably redirected to a webserver which does not know your session. ( URL does have /csrfAttackDetected.html in case when the issue happens)