SYNCING ERROR: you ave reached the maximum number of songs that can be made available offline. To get the pending songs you must first deselect another playlist

I'm using the current version of Spotify Premium on a Sony Xperia Z3 running 5.1.1. Today it has been giving me the message shown in the thread header. This message remains even if I switch off the 'make available offline' option or even undownload other tracks, far greater in number than the quantity of additional tracks I'm trying to download.

Additionally, I believe that the maximum number of tracks that can be downloaded per device is 3333. My number is much lower than that. And there's plenty of storage capacity as I have a 128GB micro SD card installed which I use for music and which is less than half full.

So it seems whatever I do, I can't download any more tracks. Does anyone here know why this is happening or how it can be fixed?