Authentication GUI only appears once per session (probably policykit issue)

For several weeks now, a GUI to authenticate (enter my password in order for sudo to work) appears only once per session. If I use it with Discover to update, afterwards I cannot use it with anything else, unless I run sudo in a terminal and get asked without a GUI.

Also, sometimes the authentication fails in a terminal. pkcon works, for example, to update or install packages, but while synaptic-pkexec does ask me for my password, the correct password immediately (no delay) fails with:

polkit-agent-helper-1: error response to PolicyKit daemon: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie
Error executing command as another user: Not authorized

This incident has been reported.