Photos show blank with triangle and exclamation mark

Ok, so I stopped the iCloud/photos syncing waited several minutes and started it again.

A "checking library" message displayed.

After it stopped I opened the Photos app and looked at one of the problem folders.

ALL the photos appeared in low res, then after a while, one by one, they were ALL replaced with the exclamation point warning triangle in a shaded box.

What is going on. The photos must be there, so why can't I see the full res versions?

Sorry for the delay in answering.

The problem described in my original post occur on my Macbook Pro. On my Mac Mini the same folders have a different behaviour. On the Mini only 1 photo show up in each of two folders where I have lost the most photos. No shaded rectangles appear. But they do appear in other folders, but in lesser numbers.

On the Mini, when i tap on a photo, a blank page opens and if I try to edit it I get the message saying "An error occurred while fetching a larger version of the photo for editing. Try again later" (translated from Danish :-) )

On the Macbook Pro, when I tap on a photo, only a blank page opens and nothing happens if I try to edit.

This is happening with photos that show as a shaded rectangle with a iCloud logo. On the Macbook Pro photos still show up as shaded rectangles with exclamation point warning triangles in the middle. However, there is a shaded rectangle for each photo that is supposed to be there. And when I bring up the folder on the Macbook Pro all photos briefly show in low res, only to be replaced by the warnings.

On the Macbook Pro there are some photos that show up with the iCloud logo instead off the warning triangle.

Yesterday I talked to Apple Support about an issue with my TimeCapsule drive dying and getting extremely hot.

They promised to call back today to talk about my loss of photos in iCloud and just sent me a mail that they tried to call.

However, I have NO unanswered calls on my phone, so they must have dialled a wrong number. :-(

I have discovered that some, not all, of the missing photos can be brought forth if I click on the rectangle with the iCloud symbol. Strangely when I click the back button they disappear again from the overview, only to show with the iCloud symbol.

Most of the photos with the warning exclamation point logo still do not come up when I click on them.

BUT SOME DO. So what is going on????

This post is in iCloud on IOS and Mac forums, both.

Optimization wasn't/isn't turned on.

I discovered by looking at the photo files in the library that all the ones with problems show a size of zero bytes.

I have a problem on my Macbook Pro where it crashes randomly so, even though Disk Utility and TechTool Pro 9.5 both cannot find any problems with the hard drive, something must have gone bad.

I am going to create a new Photo Library and importune missing photos from various backup media where I have found nearly alll that were missing. Wish me luck. ;-)

I rebuild the library the way you suggested, but no effect.