Hello,
I have tested your plugin user verfication at one site and there it works fine.
I am now trying to use it at a second site and here I have some trouble. Email will be sent and link works fine, but the activation key is always invalid.
Here is my last try. Hopefully you can take a look and check this. Thank you in advance:
<a href="https://mv.plus/en/sign-up-page/?activation_key=3f3459d2c698bae21fbc073e6e6b48b9">https://mv.plus/en/sign-up-page/?activation_key=3f3459d2c698bae21fbc073e6e6b48b9</a>
Site is under developement at the moment. You can test by yourself and sign up.
I have to mention that this is a wordpress multisite installation. My first site with user verification plugin is at same installation and I just duplicated the site with a site duplication plugin. But everything else works fine. Just user verification not.
I know, this is a free plugin and therefore support could be limited. I am willing to pay for you help if necessary.
So, please could you take a look and maybe it is just a small case for you.
Thank you in advance and best regards
Holger

Sorry for late reply, i am from Bangladesh, we haven't tested our plugin in multi-site, multi-site has built-in email verification system.
Regards