Attempting to sign up for newsletter on organization website results in "Sorry, there was a problem, please ensure your details are valid and try again." Have confirmed username and password multiple times (am currently logged in here successfully).
Website is Drupal 7 using this module to connect to API: https://www.drupal.org/project/constant_contact
Have reported this issue on Drupal module issue list as well https://www.drupal.org/project/constant_contact/issues/3189719
As the Drupal module you referenced was not developed by Constant Contact, reporting the issue on the module's project page (as you've done) would be the best route to obtain support. That said, I see that the Drupal 7 version of that module was built using our v1 API. Our v1 API has been deprecated and we are no longer offering support for it, outside of fixing critical bugs. While our v1 endpoints are currently still functional, we are moving towards fully disabling the v1 API (no set timeline is currently available).
My recommendation would be to consider alternatives to the Drupal 7 version of that module. The module's project page does include a version which has been rebuilt using our v2 API, however that version appears to require Drupal 8. I've also found the following module, built on our current v3 API, but it requires Drupal 8.8 or higher:
If our own sign-up forms don't quite meet your needs, it may be possible to collect sign-ups using a form created through Drupal, then use a 3rd party such as Zapier to add the contact to your Constant Contact account.
Hopefully one of these options will meet your needs!