Photos silently fails to import

FYI: macOS 13.5.1 Ventura Photos.app silently fails to import images or movies from a Finder folder in the following situations, although the files are shown in the import dialog or are deliberately selected there:

1. If some movie has a codec that QuickTime can not play, Photos silently fails to import it and also some random otherwise importable images or movies.

Examples of such .mp4 movies are: H.265 Codec ID hev1 and AAC audio, H.264 video and Opus audio. QuickTime Player can not play Codec ID hev1 (hvc1 is OK) or Opus audio which is common in Youtube movies.

2. If movies have (Photos generated) .xmp sidecars, Photos silently fails to import them. BTW, Photos fails to import the .xmp location for images.