I really thought I replied to this yesterday, but apparently not. Sorry about that.
We installed and are using Sendy v5.2.3 now. After upgrading, we did not change any configuration settings within Sendy or our Joomla plugins and we are not seeing the issue.
That likely points to a configuration or server issue on your end, but I can't say what offhand. Here's our version information:
* Sendy v5.2.3
* Joomla 3.9.28
* PHP v7.4.21
Beyond those, please double check your settings:
* In the System - Sendy plugin for your Sendy URL, List ID
* In the subscribe module settings for your API key
* In the User plugin for your API key (if you're automatically signing users up on registration).
Let us know if none of the above help. If not, we'll likely need access to the admin area to investigate further.
Thanks,
Alex