Photos complains "An internet connection is required to download larger versions of these items for creating a slideshow." I have a working Internet connection, and don't use iCloud (or any other klaut).

Have you tried these things?

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

And I wonder, in Photos Settings>General, do you have "Copy Items" checked?

Having "Copy Items" checked is normal, the default state.

roenass wrote: ... What did work, was adding the imported photos (in "Imports") to a new album. I could Slideshow that album just fine. Doing it directly on the "Imports" thing (smart album?) gives me a long stall/beachball followed by the "Internet Connection Required" alert panel.

I can use "FIle>Play Slideshow" with selected pictures in Imports, and it works as usual.

Again, what happens if you

• Make a new user and see if the same problem recurs with it
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.

I acknowledge that you don't use iCloud, which is why I didn't ask about that.

If you go to System Settings and search for Login Items, what do you find there? My list is empty, which I consider to be ideal. Having apps automatically run when you log in can cause a mess.

You could try opening your problem Library in the new User's Photos that you created. You may need to move your Library from Pictures to the generic Users folder and add the new user to the permissions at the bottom of the Finder's Info window. Then just double click the Library as the new User. That would separate the "is it the account or is it the Library" questions.

I don't understand "New user/new library: " Did you try a new Library on your own user account, and also try your problem Library on the new user account, and both changes worked? Or did change both the user and the Library, which would not let us know which was the problem?

What "workaround"

Have you used any cleaning apps?