Okular does not respond anymore, needs Qt_5.15

I have Ubuntu 20.04 running on my system and was using the pdf-viewer Okular until recently. However, now Okular does not start when double clicking on a pdf-file and starting from the terminal gives the following error message:
okular: /snap/okular/109/kf5/usr/lib/x86_64-linux-gnu/libQt5Core.so.5: version `Qt_5.15' not found (required by okular)
Any ideas how can I fix that?
I was thinking of trying sudo apt-get install qt5-default which I found on https://wiki.qt.io/Install_Qt_5_on_Ubuntu but this is the command for Ubuntu 12.10 and there it says explicitly that it will produce an error on Ubuntu 16.04. Since I did not want to break my system, I decided to post a question here before trying anything.
I will appreciate any advice on how to get Okular running again.

Remove Snap version by

snap remove okular

and install deb-packaged version instead by

sudo apt-get install okular

Try reinstalling the snap (it worked for me)

snap remove okular
snap install okular