Update: Appears fixed in 5.24c2! Thanks.
Here is how to replicate the issue. I've included a sample Reaper project that can be used instead of Steps 1-6.
1. Create a new project & a new track.
2. Set input to any MIDI input (egs., Virtual MIDI keyboard).
3. Set recording mode to MIDI Overdub. Monitoring setting doesn't matter.
4. This will make Step 5 easier: Preferences -> Media -> MIDI -> Enable "Pool MIDI source data when pasting..."
5. Create an empty 4-bar MIDI item on the track and duplicate it 20+ times (CTRL+D). The copies will be linked since pooling is enabled.
5. Create a time selection encompassing all the above items & enable looping on the transport. This is just so we can run the test for a while if we need to.
6. Arm the track for recording.
7. Hit record on the transport.
8. Let it run for a few minutes = Reaper frozen forever
How long one has to wait before the crash seems random. Sometimes 5 seconds, sometimes 2 minutes. I replicated this on two different Windows 10 x64 machines: one studio machine running Reaper 5.23 and another "bare bones" machine that has barely any software (other than OS & Reaper 5.1.8). Here is a freeze that occurred in < 10 seconds:
Reaper froze & I had to manually terminate the process after this. Because of that, no exception code or fault offset was created in the Windows Event Viewer log, only a message: "The program reaper.exe version 5.1.8.0 stopped interacting with Windows and was closed". I've replicated this on Reaper 5.23 as well.
There is probably a simpler set of steps that can reliably trigger the crash but I haven't figured it out yet.
Anyone else experience something like this? Any tips or workarounds? I'd be really glad to know if someone else can replicate it.