The only other active plugin is Sidebar Login.
Disabling it doesn't change the behaviour of the posts page.
I do have some other plugins that are installed but disabled. Membership, and s2member@Framework are the only ones I've added. Others were part of Wordpress install on my host's site.
Akismet, Hello Dolly, Jetpack, SiteWit, UppSite-GoMobile.
Reading Settings are:
Front page displays Static page (check box)-set to "Welcome to the home page..."
Posts page -set to "Select", choosing the assigned blog page throws a 404 error.
I'm not sure which Polylang settings you are asking about. There is only English & French on the site.
On the setting tab Plugins/Polylang/Settings:
Detect Browser language: yes
URL Modifications: set from content
Static front page redirect language page: yes
Media activate languages: yes
Synchronization: all checked
I can try actually deleting the disabled plugins, if that might help.
thanks,
rob