Nextcloud 21: App-specific password isn't saved

When I try to add app-specific passwords to my @Nextcloud account (on my own server, so I have access there), I get only a redirect and none is saved. This prevents me now from setting up #Thunderbird to access my contacts and calendar. A ticket is already created for this:

"Create new app password" won't save password · Issue #27600 · nextcloud/server

I hope they address it soon. :-)