Mysql login error only on some browsers - access denied for user ''@'localhost' (using password: NO)

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PHP

MySQL

Firefox

HTML5

chrome

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I've got a LAMP site where my law customers can download a file. I've got it pretty buttoned up with .htaccess and a CAPTCHA. The .htaccess denys everyone from outside the USA. I have a block of code that tracks downloads so I know how often and from where it's being downloaded. That's in a MySQL db. Nobody logs in. The credentials for logging into MySQL and storing the info about the download are in an inaccessible file. I usually use Chrome. I tested it with that, Edge, and Firefox and it works fine. But I had a couple customers this week who ran into a problem trying to do the download. One was using Chrome and the other was using Firefox. I decided to try it in Opera on my system and I also get an error. But like I said, if I do it using Edge, Chrome, or Firefox on my system, it works fine. I was logged into the customer's machine just now who tried it with Firefox and I cleared his cookies and cache. That didn't help. But it did work on his machine using Edge. Can anyone suggest what might be going on here? Why it's so inconsistent? Here's the error he got in Firefox:

Connection failed: Access denied for user ''@'localhost' (using password: NO) opera

Here's the error I got in Opera:

Forbidden
You don't have permission to access this resource.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

I know someone's going to ask for the URL for the site. I'd prefer not to post that here.

Thank you!

Avian

What I have tried:

I've tried this using various browsers