The pages validate via https://validator.ampproject.org and https://search.google.com/test/amp but when I access them with my Android phone or a mobile phone emulator via desktop (Chrome browser), I get "Sorry this page is not valid amp html" and get redirected to the forum's home page. When I try with any other browser (via the phone), I see the amp page. When I try with Firefox (desktop), then error is displayed again and redirected to home page.
To replicate the issue, use Chrome from a phone and search for pre-owned translatum, it will bring up an amp page. Click on that and see what happens. Please see a screen recording of the process here: https://www.translatum.gr/downloads/mobile.mp4
Example pages: https://www.translatum.gr/forum/index.php?action=amp;topic=987010
https://www.translatum.gr/forum/index.php?action=amp;topic=985701.0
