Constantly being logged out of forum

[user123]

I can't login sometimes, it shows

The submitted form was invalid. Try submitting again.

And even if I am logged in, after a few clicks on forums and threads cause me to be logged out again.
I have cleaned cookies as I read there might be problem related to that but it didn't fix the issue. Also I use Cloudflare WARP.

[XnTriq]

Sorry for the late reply, user123! I have a similar problem