Every map I've ever favorited got unfavorited.

Genuinely no idea what caused that, and also that has never happened before. Before they got "wiped" 1-2 days ago, I had around 30 favorite maps. Recently I started playing daily challenge on osu!lazer but I have no idea if it caused that. If any info is needed then please ask, no idea what info could be useful. Thanks in advance!

osu! version: Stable 20250401.2 (latest)

Talking about the favourite beatmap section on web profile?
Or was it a collection in the game? If so, stable or lazer client?

Talking about the web profile. If you go on my profile you'll see that I have 0 beatmaps favourited.

Check the database and checked an older snapshot from some months ago. You only have one favourite recorded (beatmapsets/1314466#osu/2726744).

I might be wrong but I've never played or have seen this beatmap in my life. I found out that Wayback Machine has saved my profile and sure enough, in 19 Apr 2022 capture I did have 16 favourited maps.

On a second note, might this be related to a nickname change that I did about a month ago?

It can't be related to a nickname change. The only way this could happen is if you removed the beatmaps from favourites yourself.

Now that you've found a wayback link, you should be able to add the favourites back if you choose to. So I think we can call this resolved?

I'm still quite curious about the cause but yeah sure, we can mark this as resolved.