Please test proxies inside some SEO tool by actually using tool.
So, for one single tab and one single page browser use a lot of threads - not just one.
That is why smallest "test" package of 10 threads is not enough for regular surfing in browser or any bot that use browser."
Tried to load Spotify web and got the same issue.
This issue happens with regular web browsers like Chrome and Firefox. Actually, this problem is posted in the Spotify community, and as I understood Spotify by themselves don't know the reason for this issue.
The only thing that they recommended is to delete cache and cookies and it doesn't help neither...