redownload the newest version as I added in a bugfix since not related to this issue.
Once you do that, in your case the old template url is still present and it is attempted to be added to the overall url
ie old template URL:
wp-content/plugins/tab-slide/templates/Subscribe.php
So, to fix this, go to tab slide settings,
switch template pick to anything else but subscribe or custom, and click save
the new url shold be saved now and when you click custom you should see /templates/TEMPLATE_YOU_CHOSE.php
as the new URL in the “Window URL” input field
if you don’t see that edit it and make it for eg:
/templates/Subscribe.php
if you do you can switch back to subscribe and it should work then