When I try to recover a forgotten password, this does not work when I try to use my mail address. However, it works when I enter my username. I triple-checked the mail address (the one in my profile, I was logged in with another machine), it is correct.
It seems there are two login systems, plugins.gradle.org (Gradle - C-Otto (Carsten Otto), which uses a VERY old mail address I haven’t used in years), and discuss.gradle.org (https://discuss.gradle.org/u/c-otto/preferences/account, which uses the mail address I tried to recover with).