Hey,
Sorry to hear about your troubles.
I would be more than happy to see the screenshot of the error message and talk through it/investigate it.
I can say confidently that this plugins code doesn’t touch passwords. Nothing in it can and I would be happy to go through line by line and explain code if you feel like something in this plugin is editing your passwords.
What you describe sounds like your server has been hacked or infected with malware. And because of that, I have to strongly urge you to consider that removing this plugin won’t/hasn’t resolved the entire issue.
I’m happy to see the error message, I can guess that with a malware infection multiple things could break in the admin of your site and create errors.
But if your passwords are getting changed then I suggest contacting your hosting provider. They may offer free malware/hack recovery. Or have a path for you to follow.
I personally would suggest changing passwords for your WordPress account, database, FTP. And I would make sure there are no new admin users in the site.
I suggest restoring from a backup if possible, else reinstalling every theme/plugin and then reinstalling WordPress through the updates page in your admin.
Next I would suggest installing a security plugin to help monitor file changes and run a scan.
Please let me know if I can help, and send along the screenshot
Ben