YouTube Music uploads not accessible/listed

Ignoring the language use, any chance the files are saved as .wma files?

More than likely that is a function of YouTube's implementation of the Sonos API Music (SMAPI).  As such, you will have better luck asking YouTube, because Sonos has no control over how YouTube implements their code to interface with the Sonos app. 

Not a perfect solution due to Sonos bugs, but have you considered a local music library?

Most computers can host one but a NAS is usually a cleaner and less frustrating solution. A fancy NAS will work, or an inexpensive Raspberry Pi one, which is what I use, an older and less expensive Pi 3 ($35) is good enough, even a Zero 2 ($15) will serve four losless streams well if you don't want a (not really necessary) wired Ethernet connection.

https://www.raspberrypi.com/tutorials/nas-box-raspberry-pi-tutorial/

The Pi 4 8 GB ($165 -Ouch!) suggestion there is for serving files to devices that need far more speed than Sonos so the cheaper options will serve well.

The Pi uses a micro SD card to run from, putting your music on a USB drive to keep code and data segregated is a good idea. A thumb-drive or a USB connected hard or ssd drive also works if you have a spare.

The relevance of the local music library is fully dependent on how many songs you have on your PC, Mac or NAS. If you have more than 64k songs the local music library is out of the question - that’s the max Sonos supports.

If you like your own library in the cloud for Sonos and also on the go, I recommend iBroadcast service. Works quite well for me. 
In the past I used Google Play Music, too… but after it switched to YouTube Music the Sonos integration is a nightmare. 
Like TO said, the lists of your uploaded content isn’t shown completely, and on top you aren’t able to scrub within a title. Trying to do so most time ends up in a dropped stream.

But the reason might be I‘m using YouTube Music free without any subscription and YouTube gives just limited server performance to these users.