I invested today in updating to PSE2020 from PSE13. On first launch after installation I asked to update my old catalog. After a considerable time the update catalog process froze while trying to optimize the thumbnail cache, or so it said. So I killed the process and tried again. Same thing. So I went into my old catalog folder, "hid" all the .cache files, and tried again.
That worked better, and I could then launch the program and see some of my old media. It seemed to be rebuilding the the thumbs cache in the background, but with my 45000 media files, perhaps 90% photos and 10% short videos, it was taking a while. No problem. But then after an hour or so it was hard stuck again: 10% CPU time, 500Mbit/sec network usage (my media files are on a Samba server), no UI reaction, and no change in file sizes or time stamps in the new catalog directory. I killed the process and restarted it, but it just reached the freezing point sooner.
I turned off face recognition and so on, but it didn't help. A few of my videos are HEVC (H.265) compressed (Apple switched iOS to HVEC recently) but I had installed the $0.99 official Microsoft Windows 10 (amd64) HEVC Video Extensions (and removed the "HEVC Video Extensions from Device Manufacturer") already.
I imagine PSE2020 is stumbling over thumbnailing some media file, but user feedback about what it's doing is ... shall we say ... minimal. I thought perhaps it might record in some log file what it was doing when I killed it, or when it got stuck, and I know how to rummage around in (a copy of) catalog.psexxdb with sqlite3, but the only hint I could find was in AutoCreationsCrashMonitorConfig.xml, which mentioned failedAttempts=1 with currentMedia="1105411". Unfortunately the sqlite media_table doesn't have a row with id=1105411, so that's a dead end.
.... time to think ....
Thanks to the wonderful free tool ProcessMonitor.exe from Mark Russinovich I know what happened. The process named PhotoshopElementsOrganizer.exe was reading the first 9.7MB of a 16.6MB file ...\atMG_2455.MOV, created on an iPhone 7 in May of 2019, over and over again, ten times per second. The .MOV file is a 5-second HEVC clip that plays perfectly in VLC. The only interesting thing about it is that VLC says its frame rate is 240 frames/sec; it might have been taken in slow motion. I'm happy to send that file to Adobe so they can figure out how it crashes the thumbnailer.
Anyway, I killed the process, hid the offending MOV file, and started the process again. After several hours, the thumbnails were all regenerated and PSE2020 seemed to be operating almost as expected, finally!
I say "almost" because when I exit PSE2020 normally, via File > Exit for example, the PhotoshopElementsOrganizer.exe process simply switches from foreground to background, continues using about 18% of the CPU, and it never exits. If I try to re-launch, it says I've already got one running, but there's no UI. The only recovery I've found is to kill the (now backgrounded) process from the Task Manager, or to reboot. C'mon Adobe, you can do better than this!