Do other webapps have same issue, I guess a comparable one to test could be gdevelop as that's web based.
No other issues with network, downloads, websites?
Can join online games fine?
Speed tests reporting normal speeds?
Tried different servers on the vpn to maybe bypass this issue (assuming you can pick various countries and such)?
I did have a look online as I'm sure you have already done, very unusual and seeing repeated "clear cache" which of course you did early on. Hmm.
I found another suggestion online, not sure how comfortable you'd be to try, nor can I advise exactly, but you could analyse your network traffic with Wireshark and see if your requests are malformed. It's tricky to explain further, not sure what you'd be looking for, or how would you know what traffic should look like if it is indeed malformed.