I "solved" my issue with the WebUI. After reading some of the latest posts regarding the memory leak issues I decided to install a fresh container. After shutting down my old container I renamed the appdata folder. Then I changed the container repository from linuxserver/unifi-controller:LTS to linuxserver/unifi-controller:5.11.50-ls40 (as proposed) and installed my new container. Now I could reach the WebUI! I then restored a backup from a year ago (2019-05), yes a year ago. And first everything was good, I signed in with my usual user/pass, started looking around (no changes made!) and suddenly I lost connection to the WebUI. I could not get it back. I fail to see the logic here. A new fresh container and a backup from a year ago, well before my issues started. First working, then not. Anyway, I removed the appdata folder and started from a clean install again. This time I did all, or at least all necessary, config manually to get my wifi back online. Now it has been working for at least 24h.
Hopefully this helps you other guys having this issue... maybe you are more lucky with your backup, if you have any.