I read on the mullvad subreddit that US servers don't seem to have this issue. It's still a PITA for those affected. If I have to switch to a US server, the latency is so high, I might as well just use Tor 🤣
Discussion
Mullvad has the issue that their IP addresses are well known and they are blocked by many services. Maybe because the services want to know your real IP address or their systems see to many connections from the same IP address as attack. There can be many reasons why Mullvad doesn't work