Hello! I can't make changes to the post on behalf of the administrator. In the admin console, changes to the message are not saved.
When editing a topic, the EDIT button is not active and you cannot edit the message.
This is either, you have a JS error from other plugins or this is a cache/optimizer/minifier/defer plugins affection. First make sure forum page is properly excluded from cache/optimizer plugins, purge their caches and disable them temporally for testing purpose.
If it doesn't help then you have JS errors, the errors should be debugged in browser console tab (press F12 to open) and you can see the source JS file which makes the errors. That file will point you to the problem maker plugin. We could help to debug JS errors if you leave a URL to forum, but first please go for cache/optimizer plugins checking and let me know the result.