sound – Speakers stop working after plugging/unplugging headphones – Ubuntu 20.04.1 LTS

i updated my laptop to Ubuntu 20.04.1 LTS with XFCE4 last year. Since then I’ve had the following problem:

after booting the machine the speakers function correctly – i can skype, listen to films, etc.
However if i plug in a pair of headphones and then unplug them again, the speakers stop working until after the next reboot.

Does anyone have an idea where the problem might beß

Thanks in advance,
Martin.