High Sierra: unable to open HFS disk images: " can't be opened because the original item can't be found"

I upgraded my MacBook Pro to High Sierra a week ago and have been using it without any glitch.

Until today. I have a couple of disk images, and I wanted to open one of them. When I double-click the dmg file in the Finder, an icon appears in the side bar as usual - but when I click on it, I get this error:

"<disk> can't be opened because the original item can't be found"

The disk appears in /Volumes and I can see all the content from the command line. Also from the Finder when I go to /Volumes. The directory structure is correct, but the content is not: files are unreadable.

I then tried creating a new disk image using the disk utility and I get the same effect: the image just created is unusable.

After a bit of googling, I came across a discussion about the new APFS file system: the upgrade converted my SSD from HFS to APFS. But the discussion also says that I need to convert all drives from HFS to APFS. So I did that, and my disk images are now usable again.

This is a bit of a worry: disk utility lets me create disk images in HFS, but then High Sierra is unable to use them ... Bigger worry: any disk image created in HFS cannot be read by High Sierra. And vice versa: obviously any disk image in APFS cannot be read by earlier versions ... So much for upwards compatibility.

I thought that the requirement for migrating to APFS was only for SSDs ? My external drive (the one I use for Time Machine backups) is still in HFS and everything works fine. So it looks like High Sierra is able to use HFS hard drives - which is good. But it has problems with HFS disk images ?

Anyone faced th

What does this mean for disk images used for installing software ?