Instead of seeing my photostream I just saw placeholders from Flickr...
"Image currently unavailable".
But clicking on one of these brought me to the correct flickr image.
I checked the page source and saw that the thumbnail urls ended with _m.jpg_m.jpg
Removing the last _m.jpg revealed the correct thumb.
What did the trick?
In the file plugins/flickr-photostream/flickr-photostream.php on line 233 I replaced
I am aware that this is just a quick fix.