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,