When I try to login to ACP I get this error:

The only change I've made is to upgrade PHP 7.0.15 to 7.0.16.

Try another browser

Same situation.

Edge:

Moziila:

Are you using a htaccess file to protect your ACP? It seems that it is set up, but that you're not aware of the access credentials, so it results in the "unauthorized access" error. 
You could delete the htaccess file inside of your admin directory to get access to your ACP

You need to enter the .hta user/password you've setup, typically you would of added this via the ACP security center. It will not be (should not really be anyway) the same user/pass you use for normal logging in.

If you're stuck, connect via FTP or your hosts File Manager and remove the .htapss (or similar) from within the /admin directory. Ideally download it then delete it rather than just delete it. That will remove that extra 'popup'

 

Just now, Daniel F said:

Are you using a htaccess file to protect your ACP? It seems that it is set up, but that you're not aware of the access credentials, so it results in the "unauthorized access" error. 
You could delete the htaccess file inside of your admin directory.

Beaten to it.