Hello @tsvetanm,
Thank you for responding promptly.
I deactivated all the Frontend optimizations (JavaScript/CSS/HTML) to see if I can add them back one-by-one. I even cleared the cache just in case before trying to submit a message in the Activity page. Unfortunately, even will all the Frontend optimizations, the issue remained.
I am a SiteGround customer. I will submit a support request directly from the Client Area.