You marked this older topic as resolved, but the problem is present in the latest version (1.1.14).

Your plugin is still passing POSTed data through sanitize_text_field(). The purpose of that function is to escape text for output within HTML, and has nothing to do with sanitising inputs. First, start with a clean copy of POST data like this:

$post = stripslashes_deep($_POST);

Then use values from that array. For credential inputs, the most you should do is trim(). Passwords are always best left as-is.

Currently, a perfectly normal random password like 1Q,z<_%39P would be “sanitised” by your plugin to 1Q,z<_P.