Endless authentication loop entering AdminCP

[JimMM]

When attempting to log into AdminCP we use the following steps and get these results

Log in with admin userid and password
Turn on Site Edit
Click on AdminCP button on far left of menu bar
Enter password in the AdminCP secondary authentication window
The window refreshes with both user id and password blank
Re entering both values just repeats the process

We have found a workaround to click the on the forum home link in the looping credential window
re log into the forum with admin credentials.
the menu bars comes up with site edit enable
Clicking on the AdminCP button on the far left let's us in with no popup authentication window.

Deleting all browser cookies doesn't seem to change this
Tried IE 11, Chrome 63 and Firefox 57 Windows 10 FCU
mySQL .6.36-82.1-log
PHP ver 7.1.12
Linux
Apache (cgi-fcgi)

this is at https://forumania.com/forum

[In Omnibus]

Is your secondary authentication redirectiing to http instead of https?

[Mark.B]

Are you using Cloudflare?
If so exclude the admincp in Cloudflare's settings.

[JimMM]

No, it doesn't ... it keeps looping at https://www.forumania.com/admincp/index.php and it loops at the same place when logging out of AdminCp, keeps asking for the admincp userid and password.

No, we are not using Cloudflare.

I don't know if this is the cause or not, but we have an html page which an .htaccess file reroutes our domain to, which serves as a quicklink index to all the forums. It uses https://forumania.com and forces https::// The actual top page is at https://forumania.com/forum or /index.php

[EDIT] I just read another thread with a similar "endless loop" issue, so I checked both cookie prefix's and they are the same:
$config['cookie_prefix'] = ''; Should this be a non null value?

[In Omnibus]

You should exclude the admincp folder from any custom .htaccess redirects.

[JimMM]

I believe the only redirects we made were to the ForumCenter "front page" to he forum. I see an uncommented line referencing admincp which reads:

# Because admincp is an actual directory.
RewriteRule ^(admincp/)$ index.php?routestring=$1 [L,QSA]

I've atttached the htaccess text version as a link to the dropbox file, hope it works

Hmmm, the link looked ok in the link box. maybe this will work ....

https://www.dropbox.com/s/tb44bzd8qft6n1w/forumania%20public%20html%20htaccess%20file.txt?dl=0

[In Omnibus]

The link goes to the Dropbox index page, not to a specific file.

[Mark.B]

Check both config files and make sure the cookie prefix value is identical in both. It is 'bb' by default.

[JimMM]

In both config files thed cookie prefix value is null or empty, i.e. nothing between the single quote marks: ''$config['cookie_prefix'] = ''; Should this be a non null value?

Do you think changing the prefix from nothing to bb have any effect on the endless loop problem?

[Mark.B]

You should have a value in there. If they are empty, someone has removed the values.

Set them both to bb and see if the problem is fixed. It is essential that the value is identical in both files.

[JimMM]

Thanks, we're sorting out some other problems and I have been asked not to change anything right now. I'll let you know when I can change it to bb and test.