Making the Posts Page work

what theme are you using?

Twenty Ten.

temporarily switch to the default theme Twenty Fourteen to see if you are able to set the posts page there.

if not, temporarily deactivate all plugins to see if that helps. if yes, reactivate one plugin at a time and test inbetween to locate the interfering plugin.

also, enable DEBUG to see if you get any error messages – https://codex.www.ads-software.com/Debugging_in_WordPress

Thanks. This problem has now resolved itself, first in Firefox and now in Opera.