The screenshot below left shows what happened until recently when accessing plus.google.com, the screenshot below right shows what happens now. And it's to do with the EdgeHTML engine built into Windows 10 Mobile, which all the third party 'browsers' use, so it doesn't matter which one you try.
Any tech experts know what exactly is causing this? I doubt it's something malicious at Google's end, probably more that they changed something in the HTML5 code and no one checked it on a Windows 10 Mobile handset.
Still, it's all a bit unfortunate, even though the Venn diagram intersection of Google+ users and Windows 10 Mobile users is vanishingly small (possibly just me!)
I thought it was worth noting anyway, in case any web experts can shed some light on this. I can update the story if a workaround is found. Or if Google's web code magically starts working again!
PS. I tried Internet Explorer under Windows Phone 8.1 and got the same error.