invalid security token
 

When I click to mark forum read shown that warning:

Your submission could not be processed because a security token was invalid.

If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error.

What must I do?

Thanks.

Many threads on this, go search for them. See the sticky by Zachery on vb.com

<a href="https://www.vbulletin.com/forum/showthread.php?350975-Invalid-Security-Token-Error-on-Mark-Forums-Read" target="_blank">https://www.vbulletin.com/forum/show...rk-Forums-Read</a>

Basically 3.8.5 added a required security token to mark all forums read so people couldn't be 'tricked' into doing it by malicious users, as a result templates either need to be reverted or can be manually updated with the code in that thread.