She has no trouble playing a playlist from my Plex server. yet), not a local device issue, or one related to a specific playlist. This really feels like an Alexa/Amazon Music issue (though obviously not everyone is affected. To ask Alexa for help when making a playlist. My playlist names are simple already, and she correctly repeats their name back to me while saying she can't find them, so I don't think this is the issue. To make a playlist with Alexa on your Amazon Echo device, you have the option to ask Alexa or create one using the Amazon Music app.I have several Echo devices, and each has the same issue, though I've only tried rebooting one of them. Same result after turning it off and on again.When using a routine to play a playlist, she says 'I can't find playlist X'.Though if I ask the Alexa assistant on my phone to play the same playlist, she again responds 'I can't find playlist X'. The Amazon Music app on my phone has no trouble playing the playlist.

Yes, she hears me correctly as verified in Activity of the Alexa app, and even repeats the name of the playlist back correctly while saying she can't find it.

Not OP but the exact scenario started for me yesterday as well, so I'll play along with your list.

Amazon music playlists for alexa