Thanks again, I have tried on the server side to downgrade ffmpeg and flac, but to no avail. (The reason I did was that I haven’t observed this problem earlier, so I thought maybe something had been updated that contained the “bug”).
But one thing still bothers me. How can it be that two encodings with different flac-versions of the same song causes two different behaviours when played back? I mean, if the library that decodes is the “culprit”, how can it work with one version of flac encoding? Is it that I got lucky in terms of e.g. buffer size, or seek tables, when the encoding was done?