This problem can appear due to various issues.
1) In case you are using proxies, make sure the proxies are working. To do this, click on 1 account (in accounts module) and click on Open In Browser. Click on LOGIN and see if the pages load. If the YouTube page does not load, it means the proxy is not working. Try to upload without using proxies.
2) Try to delete the cookies. Go to accounts section, right click on account and hit delete cookies. Then try to upload again
3) Always watch the browser from the bottom (click on SHOW BROWSER) and make sure that login process finishes with no problems. If the login process fails (a timeout error) you will need to login manually. To do this, go to Accounts section. Click on the account and click on Open In Browser.
After that, click on LOGIN and let MVB Pro login to your account. In case any captchas appear or the login process stops, LOGIN MANUALLY. Once logged in, click on FORCE SAVE COOKIES From top and then start the upload process