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I noticed the issue with comments few months ago. Given the fact that it takes years for cgtrader to fix serious and even critical issues, i wouldn't hold my breath for this to be fixed anytime soon, but if you want to have higher chance for comments problem to be fixed, report it straight to support instead of using the forum - they hardly visits here and might not be aware of the issue, unless someone will report it directly.
Until last week, I had no problems posting comments. It was strange when it suddenly stopped working, and I wasn't sure if the site had undergone updates or if it was related to a recent browser update I installed. I attempted to troubleshoot the issue but couldn't identify the exact cause, aside from a potential faulty "callback" in their script. I plan to report this directly to them, but I share your frustration with how slowly these minor issues tend to get resolved on these platforms.