You don't say whether to problematic images have upper-case letters..., and you don't say which version of DokuWiki you are running, whether the theme/template is the default (unmodified), or whether all your plugins are up-to-date.
So downgrading to Adora Belle allow you to view the images in DokuWiki and Media Manager.... my
guess is that you have a conflict with some plugins, or you have (perhaps unintentionally) modified the default theme and/or configuration of DokuWiki.
First check the permissions on those images. If they are the same as images that do show, continue testing...
1. I'd backup the entire wiki either at the CLI if your server runs Linux and you have ssh or local access
tar -zcvf $archive-name.tar.gz $directory-name
replace
$archive-name with something descriptive, replace
$directory-name with the full path to the directory containing your DokuWiki installation, run the command from the location where you want the archive to be.
or - use your web-management interface (cPanel etc), or use an ftp manager (e.g. Filezilla)
2. Upgrade DokuWiki to the latest stable (unless you have a compelling argument not to)
3. Test (and check if the image is available in Media Manager to rule out cache problems). If those images still don't show, continue....
4. Disable
all plugins.
5. Test. If those images still don't show, continue.... (otherwise enable plugins 1x1 testing after enabling each one)
6. If you still can't view those images - double check that you are using the stock, default theme (reinstall if necessary)
Please post your results. If you can't fix the issue please include information about your server caching (including whether you are using a CDN), and whether
you installed DokuWiki or whether it's provide by your hosting service.