After installing vanilla-2021009 normally, the topic is wrong

wbcm

wbcm

The problem is found, it is caused by Rich Editor

Kaspar

Problem is due to pretty urls not being enabled.

You got index.php in your url.

Depending on your hosting system, follow one of these

https://open.vanillaforums.com/discussion/comment/220157/#Comment_220157

https://open.vanillaforums.com/discussion/27568/enabling-clean-urls-on-iis-installations