Judging by the looks of the error, it is not the Mendix runtime responding, but some other web server. Probably, you have installed another web server since August, which uses port 8080.
You can easily run Mendix on another, free port to see if that works: Project>Settings>Active configuration>Server (tip: change both runtime and admin port). The run again (tip2: use the View button).
I would expect Mendix to throw an error in the Console as well though if the port is in use already.
What happens if you only access the URL localhost://8080?
Thanks a lot for your fast and constructive replies!
And @Lucas Meertens, you nailed it. I installed Docker Desktop in September, which was I thought was only using 1880, but it also used 8080. In the Mendix Console there was no error message by the way, I don´t exactly know why…
After stopping Docker Desktop, it works fine.
Again, thanks to everyone – you´re doing a great job!