Separating this by last.fm issues and Spotify issues:

Last.fm
I've tried putting my email/username into the Client ID field and my password in the Client secret field, but the webpage I get sent to shows this:

The URL in this specific instance is https://www.last.fm/api/auth?api_key=username&cb=http://localhost:45305/ - is this the intended behaviour?
(note: running Strawberry via CLI with strawberry --verbose shows no errors, warnings or any other kind of feedback in this instance.)

Spotify
No matter what I put into the Client ID and Client secret fields, it always returns

ERROR SpotifyService:259               "Missing Spotify client ID and/or client secret"

and I am never redirected to the Spotify website.

If there's an API key I need to fish out from somewhere and put in these fields, please let me know because the services themselves don't show me anything (unless I'm just real bad at searching :^) )
Strawberry is my music player of choice for both local and streamed tracks, so I'm very happy to help troubleshoot any issues and get the whole setup working again! Thank you for your continued hard work on this wonderful music player. ^^

Additional edit:
I just noticed that there's a place to log in for Genius too, so I tried that and also get a webpage saying "Invalid Authorization". The URL is https://api.genius.com/oauth/authorize?client_id=username&code_challenge=LW6f0eupn2UFYo47mgoaIidHdcLlqC8--LGQVnxy44Q&code_challenge_method=S256&redirect_uri=https%3A%2F%2Foauth.strawberrymusicplayer.org%2F&response_type=code&scope=me&state=qwx1CBcGJHvPC6wtoXjhLRSbsvZEVIpM-36917

If this is something I should add as an issue in the GitHub repo, let me know and I'm happy to open an issue there too