I recently published over 5,000 photos to Flickr with LR 5.6 and got a similar error message several times, "function flickr.photos.getInfo, message Photo not found". Luckily, I was able to restart the publish each time (and then painfully examine the uploaded photos to discover which few that LR thought had been successfully uploaded but hadn't actually made it -- I can give you a trick about how to do that reasonably quickly, if you want).
You might consider switching to this third-party Flickr plugin. The author has an excellent reputation and actively maintains his plugins (in contrast to Adobe).
Thank you so much for your reply, I'm a amazed that Adobe does such a poor job on this, I paid good money for that product. I'm curious about your trick to make this process faster. Maybe I will end up using the plugin you recommended although I always feel a little bit uncomfortable with using unofficial software.
Thanks again for taking the time to answer my question.
painfully examine the uploaded photos to discover which few that LR thought had been successfully uploaded but hadn't actually made it -- I can give you a trick about how to do that reasonably quickly, if you want
Put LR in one half of your screen and the browser in the offer half. In LR, select the Flickr published collection in question. In the browser, go to Flickr's Organize > Albums & Collections and double-click the corresponding album. In LR, use the Thumbnails slider to adjust the number of thumbnails displayed, and adjust the width of the browser window to adjust the number of thumbnails that Flickr displays, so that each is showing the same number of thumbnails per row. Then you can quickly skim down the rows looking at the first thumbnail in each row to ensure it's the same. When you get to a row with different first thumbnails, the missing photo is in the previous row. (Click on the screen shot below to see what it looks like.)
Once I figured this out, it only took 5 minutes to find the 5 photos out of 5000 uploaded that didn't actually make it to Flickr.
That's a very good tip. Thank you so much.