The password should normally work unencrypted. The only exception is when the password itself is base-64 encoded and has specific length (44 or 88 characters) - in this case NetXMS will think that it's something encripted by nxencpasswd - in this case the workaround is to actually encrypt it with nxencpasswd.
Also, double-check the "Login" field.
Also, double-check the "Login" field.