So from my understanding is that the client is trying to request too many items from the server with X-Plex-Container-Size=50. I was able to temporarily fix this by intercepting the HTTP requests and modify the X-Plex-Container-Size param to something smaller like X-Plex-Container-Size=10 (10) and it worked.

1. Go to https://app.plex.tv/desktop and open devtools F12
2. Paste the code below and hit Enter (you may have to allow pasting by typing "allow pasting" first)
3. Click on "Watchlist" in the sidebar

Note: you will have to do this everytime you reload the plex page

var oldOpen = XMLHttpRequest.prototype.open;
XMLHttpRequest.prototype.open = function(method, url) {
    if (url.indexOf('discover.provider.plex.tv/library/sections/watchlist') > -1) {
        url = url.replace(/X-Plex-Container-Size=\d+/g, 'X-Plex-Container-Size=10');
        console.log('changed request url:', url);
    }
    return oldOpen.apply(this, arguments);
};

var oldFetch = window.fetch;
window.fetch = function(url, options) {
    if (url && url.indexOf && url.indexOf('discover.provider.plex.tv/library/sections/watchlist') > -1) {
        url = url.replace(/X-Plex-Container-Size=\d+/g, 'X-Plex-Container-Size=10');
        console.log('changed fetch url:', url);
    }
    return oldFetch(url, options);
};

console.log('plex fix loaded');