There is no cape nor is there an error.

Running 1.21.4 on fabric, and no incompatible mod is installed. (I had a visit on the modrinth site)

I mean, which cape should be there/where from?

I thought my optifine cape or an cape from official minecraft

Then you don't have them equipped or enabled. WaveyCapes doesn't add capes, so they need to be visible without the mod too. Check in the launcher that you have your official cape applied, and also in the skin settings that the cape is turned on.

Thanks for the tip @tr7zw
Indeed the cape was disabled on the website. Enabled it and now it works fine.