Error with a proxy: "Cached Preview needs 2 or more frames to playback."

To answer an earlier question, yes, Comp Proxies are supposed to make previews faster as AE is just loading the frame from the rendered proxy file rather than rendering the frame of the layered Comp and loading it into RAM.

So, something is causing an unexpected 2-Frame playback error.   (It, of course, should only appear if you've set the Work Area to one frame.)

I was thinking it could be a version 14.5 bug, but I just tried setting a Comp Proxy on my end and it's working as expected.

It looks like you're using the Lossless with Alpha Output Module (which is usually fine when setting Comp Proxies that need to maintain the Comp transparency).  You could try Apple ProRes4444 with Alpha instead.  Or, if you don't need transparency, you could try Lossless instead or switch to Apple ProRes422 HQ.  It might also be worth trying Apple ProRes422 Proxy which is the lightest weight of the ProRes options, but while it will have good picture quality it has a low peak signal-noise ratio (PSNR) and Comp Proxies should be disabled when rendering high-resolution output with ProRes422 Proxy.

You could also try disabling the Comp Proxy in the Project panel, import the rendered movie file, and then place it in front of the nested Comp that it's a proxy for in the containing Comp (also, hide the visibility of the nested Comp).  Then, see how that behaves when you do your preview.

In addition to purging memory in After Effects, you can also purge memory in Terminal.  Sometimes relaunching AE and/or restarting can resolve the 2-Frame error message when it appears when it shouldn't.