I installed MC24 (up from MC23, up from MC22 and up from MC21 all be it on different Mac OS, never any issue like this) on 2-MAC Book Pros. After restoring my MC23 database. (and yes I did restart MC24 after age restore) MC24 would freeze when I try to play a movie on the actual MAC Book Pro from within MC24. I went to the Media Network and deselected "Use Media Network to share this library and enable DLNA". Restarted MC24, re-enabled via the wizard and no more freeze ups on either MAC Book Pro! So far so good on using these 2-MBP!
On my iMac, I followed the exact same process that I used for the Mac Book Pros, however not so lucky with this one.
If I play a movie on the iMac within MC24, it will try to run the movie and then it freezes. I have no mouse control, no moving to another space, nothing.
If I leave it alone, it will eventually present me with a new login screen. The machine did not restart or reboot. It's like I was logged out. No luck with getting this to work.
If I use a browser to access MC24 on the same iMac (localhost:52199) I can see all of the movies and PLAY a movie with no issue. If I play a movie via the browser and THEN go to MC24 and play a movie, the movie freezes but the browser movie continues to play. It does not present a new login screen. I never let it run for the duration of the browser movie to see what happens.
On the iMac within MC24 when you try to play the movie, sometimes you get just the initial frame from the movie (no sound) then it will freeze. Sometimes you get 1-5 seconds of movie and sound and then the freeze.
I can play music from MC24 on the iMac with no issues.
I have Parallels VM on the iMac (as does one of the Mac Book Pro) and if I install MC24 on Microsoft Windows 7, 8 or 10 via Parallels VM, MC24 plays just fine within or via a browser.
Never had any issues with MC23 on any of the machines running High Sierra until MC24.
Each of the MACs has it's own library. But the library is the same for all of them. All music and movies are on QNAS Drives. (1-for music and 1-for movies)
My iMac is mostly the main machine that I use for MC. My Mac Book Pro (touch) is the second most used.