I think @Pickle1 is correct in thinking that this is due to switching to using a P2P (peer-to-peer) network for the preview clips and that the solution is to disable P2P on the settings page.
However, from memory, in the past this used to show up as the download apparently hanging (or just being very slow) with the download progress being displayed as having reached the 100% level rather than 95% - but this difference may be due to a change in which the way the previews contribution is calculated or just down to the way the percentage value is rounded.