XF 2.1 HTTP403 Error when editing style properties in v2.1.4

[Nikolaos]

We very recently upgraded from XF1 to XF2.1.4. It was a pretty major upgrade of course, but the forum is functional and our post history is intact.

However, I get HTTP403 errors when I attempt to edit style properties in the administrative panel. I can perform other actions such as assigning permissions to users, editing templates, and so on, but style properties in particular cannot be edited.

This is a fresh upgrade; everything on the forum is whatever the upgrade added or left for us. I uninstalled all plugins before upgrading.

Any thoughts on causes and solutions?

Thanks.

 

[Abraham54]

What PHP-version runs on your server?

 

[Nikolaos]

What PHP-version runs on your server?

It seems to be 7.3.6

 

[Abraham54]

My forum runs on 7.3.11.
But in my eyes PHP should not be the problem.

Have you possibly Varnish-cache active?

 

[Paul B]

It's going to be a security mechanism on the server - something like ModSec.