It looks like I had two different problems.
1. The Veronica Top 1000 aller tijden 2010 playlist appears to be a broken playlist.
2. It looks like there is a hard limit of 500 songs in the playlist size, set by MySqueezebox.com, which means it cannot be overridden by the setting on the LMS.
I solved it by ditching the official Spotify plugin.
The Spotify 2.2.4 plugin by Triode has the possibility to set the maximum playlist size.
Now it works like a charm.
I'm very happy it works now.