Since the DST Root CA X3 root certificate that Let's Encrypt uses expired on September 30th 2021 (see https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/), video URLs hosted on servers that use Let's Encrypt SSL certificates will fail with the following error. I was able to work around it by manually installing the ISRG Root X1 root certificate from https://letsencrypt.org/certs/isrgrootx1.der
2021.10.06 13:27:46 Log - NativeProcess.Start: started process id [4552]: C:/Users/XXXXX/AppData/LocalLow/VRChat/VRChat\Tools/youtube-dl.exe (...)
2021.10.06 13:27:47 Log - NativeProcess.HasExited: process exited with code 1, took 998 ms. Command line: C:/Users/XXXXX/AppData/LocalLow/VRChat/VRChat\Tools/youtube-dl.exe (...)
2021.10.06 13:27:47 Error - [Behaviour] ERROR: Unable to download webpage: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1076)> (caused by URLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: certificate has expired (_ssl.c:1076)')))
2021.10.06 13:27:47 Log - User XXXXX added URL https://xxxxxxxxxx.xxx/path/to/video.mp4
2021.10.06 13:27:52 Error - [Behaviour] Can't play movie []