Version 1.16.5 - I don't know why I couldn't choose it on the list??

In one second I can see my skin (it is visible for ca. 2 seconds) and in another... I can't! Why it can be like this? Skins like Aleks or Steve are able to load but not mine skins :< 
Earlier I was able to use them (before I reinstall the game), and now even reinstall didn't work  Please help me