On 4k clips now and using the hardware encoder (h264_qsv) to proxy clips and a file errored.
Meanwhile I was having windows upgrade issues ie KB5034441/0x80070643. I found help on t’internet to fix the upgrade.
I tried to create proxies again to capture the errored ones to proxy sans h264_qsv and surprisingly was able to proxy everything with the h264_qsv.
I’m therefore linking the upgrade problem with the proxied errors. Hopefully anyone with similar issues can find this fix for them 
Oops! What do I know? LOL!
I was looking at the post by @SimPaulJack on the issue this year with proxy fails - because my issues with hardware encoding proxies (and possibly exports too) has returned. Do you think it is to do the upgraded ffmpeg that’s causing this, even though it’s fixed for 24.1?
I too used to be able to use the hardware encoder on proxies, but looks like it may not help that much to even use it. Might be something that should just be removed until it either becomes significantly faster to use or at least more reliable to work all the time. Cut down on forum posts complaining about it at least 
If you are posting here about using the beta, it is expected that you read the release notes before posting about an issue. It says
“Fixed proxy generation failing with NVENC if Settings > Proxy > Use Hardware Encoder is on.”