We don't have such line on 3306. Also, this function was not changed for last year and it's impossible that you have this on 7.6.9.5 and don't have on 7.6.9.3 

Provide admin access

Please don't be so dismissive of my issue.  The problem lies within the REHub theme template files.

The theme is "REHub 7.6.9.5" with "Dokan 2.8.5" (free) and "Dokan Pro 2.8.3" (purchased) and "WooCommerce 3.4.5" -- all of which are the latest available.

Note:  The problem only exists with "REHub 7.6.9.5". Prior to that, I had "REHub 7.6.9.3" and it was fine.  If I reinstall "7.6.9.3" it's fine, but I shouldn't have to downgrade a theme for compatibility with the plugins its specifically designed to work with.

Also, the problem goes away if I disable "Dokan", but of course, the problem lies within the themes templates -- and it's meant to be a Dokan compatible theme.

I should also point out that there's no problem if I apply a different theme altogether.

What next?

This function is default function of dokan and it can't make issue. Maybe
you have not proper installed Dokan. Also, latest dokan is 2.8.5 Also,
maybe you installed Dokan PRO without dokan lite.
https://wedevs.com/docs/dokan/getting-started/installation-2/

If you still have issue, write to dokan authors and send me their answer

Hi.

After applying all the latest available updates for Dokan (2.8.5), Dokan Pro (2.8.3), and the latest REHub Theme (7.9.6.5) -- in a WordPress 4.9.8 site -- I get the following fatal error on the frontend:-

Fatal error: Call to undefined function dokan_is_user_seller() in [LOCAL-PATH-WITHHELD]\wp-content\themes\rehub\shortcodes\shortcodes.php on line 3306

NOTE:  This is a website being developed on a local server, so I cannot link you to it at this stage.  Prior to the above updates, it functioned fine.

Thanks.