Are you using Transmission 3.0 by chance? Some users have had to roll back to the previous version (2.94) to be able to work with the SSL tracker.
As for the blocking, there's not much we can do, as the DDoS protection is a 3rd party service, for which we are not able to configure or tweak.
I'm using qBitorrent v4.2.5.
Are you able to get a stats page when you open this link?
Yes I can.
I am now disabling the option of SSL tracker and it worked quite well at the beginning. But now the upload speed is significantly slower and I have to manually stop and resume the torrents to connect to the peers for uploading. Is this a problem with port forwarding?