Step 1: Login Yandex Mail Account
Step 2: Go to 'Account Management' (Right Side on Top)
Step 3: Create 'App Password' for accessing Third Party Tool
Step 4: Use this password for Login in Regain Yandex Mail Backup Software
Otherwise Regain Yandex Mail Backup software not able to access your Yandex emails. Here is the screenshots, where you can change setting.