This has been an ongoing issue with Spotify Web Player and a recurring issue. There will be days where the Web Player will load the playlist completely, but if the playlist cannot load completely, it behaves like that the whole day.
Clearing internet cache and cookies does not resolve the issue and the issue does not always correct itself after 24 hours. Again this is an inconsistent but persistent issue with the Web Player of Spotify.
Smaller playlists (for example, under 100 songs) seem to load just fine, but larger playlists consistently see this problem.
Also submitting this issue took several attempts because your interface does not identify that specific fields in the body need to be removed, that labels are mandatory, and that you cannot create your own labels, you have to select the pre-populated labels below that do not denote that this is a BUG, FIX, Error, Web Player (it must be assumed that is what WP means but no context. That could me Well Played or work-in-progress).
We are aware of an issue with the app and are currently investigating it. Feel free to contribute in this topic where we gather information from affected users and keep everybody up-to-date. We'll get this sorted as soon as possible.
Sorry for the inconvenience. Let us know if you have further questions.