Invalid security token error

[praveen1980]

Users in my forum are complaining they are receiving
"Your submission could not be processed because a security token was invalid".
whenever they try to login or whenever they try to download stuff (i am running DownloadsII plugin) or when they click on Today's posts.

Until sept-oct 2015, we never had this issue.

The forum is running Version 4.2.3 with a few add-ons.

I searched, applied a few fixes mentioned but they are not able to fix it.

Any idea how can i fix this? Since 2008 this site has been on a custom theme and as i said above, never had this issue.

It's been happening more regularly only in the last 30 to 45 days.

thanks.