For many months now, I've been running a Windows 11 21H2 virtual machine on Parallels with no issues whatsoever. I didn't bother updating to 22H2 when it came out, but updated to 23H2 about one month ago, after making sure I had a valid snapshot of 21H2. My first update attempt was disappointing, as Windows 11 said my computer wasn't compatible with 23H2. Perhaps that was to be expected, as my Parallels runs on an old Mac Pro 5,1, whose processor lacks, for instance AVX/AVX2 instructions and the like. Next, I tried to force the 23H2 update by resorting to an ISO DVD installer. This achieved installing 23H2, but then Windows would crash before reaching the Desktop. Further tests proved that 23H2 would crash or start-up if Core Isolation was off. So, I went back to my 21H2 snapshot, set Core Isolation on and reinstalled 23H2. This time, the updated Windows 11 23H2 booted successfully, but it was slow as molasses. After creating a 23H2 snapshot, I set Core Isolation off and then booting was impossible once again. Over the next couple of weeks, I changed several things in Windows 23H2, including one or two registry keys. I can't recall the specific things I did, but the end result was (and I made sure a new snapshot was created) a configuration where Core Isolation and Microsoft Vulnerable Driver Blocklist were off. This allowed 23H2 to run smoothly on my virtual machine. It even allowed for Windows updates to be automatically installed, provided Windows itself did the entire restart process, WITHOUT MY INTERVENTION. The two or three times I tried to restart the virtual machine myself (by going to Actions/Restart in the Parallels menu), the result was a Windows crashing and the impossibility to reach the Desktop. So, I went along without initiating a restart myself. A couple of days ago, however, I upgraded Parallels to version 19.2. This entailed updating Parallels Tools on all my virtual machines. I didn't have any issues with my Snow Leopard, Mojave and Monterey virtual machines, but when I opened my Windows 11 23H2 VM and the process was initiated to automatically install Parallels Tools, several consecutive attempts ended up in crashes. Therefore, I went back to my up-to-date 23H2 snapshot and when Parallels tried to install its Tools, I postponed the installation and went to bed. This morning, much to my surprise, the Windows login screen was waiting for me to enter my PIN. After entering it, I reached the Desktop without issues and noticed Parallels Tools had been installed alongside some Windows cumulative update. This seems to confirm that 23H2 is stable for me if I don't try to restart the virtual machine and leave all restarts up to Windows itself. I wonder if this oddity can help the Parallels developers diagnose what difference there might be between letting Windows decide when to restart (after a few hours of inactivity) and having the user initiate a restart.
