Suddenly I see that several notes do not show the attachments. While the link is still there: 
the jpg is not rendered...
This is one of several notes that has this behaviour. The note contains quite some attachments, and from a certain point in the note onward, the attachments do show.
Diving into this, I see the .crypted file with the same identifier is there.
What to do?
The version you are using. 2.7.15 (prod, win32)
If it is a sync issue, please let us know what you are syncing with: webdav on owncloud
Do you have anything that will break the rendered html? Like any open HTML tags in your markdown?
If you take that same markdown reference and put it in a different note, does it show there?
And does the resource exist, unencrypted, in your resource directory? Help > Synchronisation status will show you if there are any resources that are downloaded but not decrypted.
This is a very plain note, where I drag the images into; no plugin or html involved. The particular images that are no longer showing have been there for quite a while (a few months) and it always worked nicely. The same not contains images that do show.
I thought I had one or two more notes with this problem. In any case the note has been there for a while.
Could you search in your log.txt if there's any reference to "67581c1ee572e1f8b0f1d383421792c9" or "Xeroxscan20181130006.jpg"? It's under Help > Open profile folder
I encountered a similar issue on 2022.03.05. Filing an issue is on my backlog, then I came across this post. My intention is not to hijack this thread, nor distract from the issue of the original poster here. Commenting in the possibility that my logs could help triage our issues.
I intended to post to GitHub, but as the troubleshooting has already started here...let me know, and I'll post on GitHub to track better.
When the issue happened, I deleted the text from my note, as it didn't connect with a file and I considered the file lost for the moment. But I extended my note history anyway, just in case I could recover what was lost, after reporting the issue.
Today, I grepped the string to find the matching .png and .crypted files were still present in joplin-desktop/resources.
I recreated the link from the screenshot. And it worked. The text becomes a working link that opens the file.
I still have no idea what happened. Maybe the file hadn't been present in resources, but only temporarily. I'm glad I didn't actually lose the file, but happy to help dig. Again, there may be other instances of this issue elsewhere in my notes, but I don't know how to find them.
I have/had a very similar problem: Quite a bunch of images did/do not show anymore in my Joplin Portable on Windows10.
The link was still in the markdown, the preview showed it as if the image was missing (the small symbol that browsers show).
The image file with the resource number of the link could be found in the ./resource folder with its proper size, as expected. But:
The file could not be opened anymore with any image viewer.
Reason: It was fill with 0x00 instead with the image data.
This applies to all the images in notes, which I do have this problem with (quite a lot actually!).
Up to now I do not have an explanation on how this happened.
Recently I found out, that (at least some of) these images that are corrupted in the desktop version are still available in Joplin on my mobile (Android). Apparently the mobile apps database data about these images, in which they seemed to be stored, has not been overwritten by the corrupted data. The same image files on my nextcloud that I use for syncing are not corrupted either.
I'm not sure I follow the story but if you removed the resource from the note, it will indeed stay in the history - however if another device, mobile for instance, has a low history retention, it might get deleted there, regardless of desktop settings.
So I don't know if this is what happened, but it could be. To figure it out we'd need to see the log on all the devices
Yes, that's right. I increased the history retention on all syncing clients, having remembered reading elsewhere on this forum that history is retained according to the shortest history setting on one of the clients. That's a win for this forum as a valuable knowledge base.
The log on other clients no longer include that time period. Apologies that my lengthy task backlog wiped out details that could be helpful here. I've made a note of this thread and will post here if I find more occurrences.
Hinny, thanks for the suggestion. Yes I have the OCR plugin installed, but only to be run manually.
As a result the note in question does not have any altered markdown...
And about the retention issue: history is set to 90 days on at least 3 (out of 4) clients.
But the note has not been deleted, so I think this is not relevant for my issue.