Hello, I installed Hotfix 10.0.44 and the issue was resolved. I can now post comments on blog posts and reply to forum with no problem.
Using free version and personal site template straight out of the box. No modules/features or settings/options were changed. I did try to turn on/off comments on blog post from the form tab and turn subscriptions for comments on/off on blog itself (neither solved the original error message).
Thank you to Pavel Jiřík who said a bug was fixed.
"Email marketing - Emails couldn't be created without an EMS license -> When using a license other than EMS, creation and editing of email campaign emails led to an 'Access denied' error."