[edit] Solved, problem and solution as follows:
Problem:
I could not access editor.construct.net/r293-3
White page loaded with info that the site might have been moved or been deleted.
Solution:
Click the small "i" icon on the far left in adress bar.
Click "cookies"
Click "delete"
Reload page
Next problem:
I could not find the option to install construct 3 as app.
Solution:
paste "chrome://apps/" (without quotes) in chrome adress bar
right click Construct 3
click uninstall
reload "https://editor.construct.net/r293-3/"
Now the option to install as app should be available.
I hope this can help anyone else with similar problems in the future.
And as ashley commented in a response, "latest stable" should be preferred for stability, but for debugging transition to newer versions this process was necessary.