I did as suggested.
and
And I followed the trouble shooting as directed too. Oh well...
Oh really? How come that I can't spot the link to your gallery and the non-admin test user account? In the upload troubleshooting section that I already have refered to multiple times (and that I have told you to read extra carefully) this is clearly explained. As you appear to be a lazy person when it comes to reading a bit of instruction that is only one click away, let me quote the upload troubleshooting section of the docs, highlighting the important part (the one that you obviously failed to read):
Upload troubleshooting
If you are experiencing issues with coppermine's upload process, temporarily change your coppermine settings as suggested below to get more detailed error messages:
* Go to your coppermine page and log in as admin
* Go to the config screen
* Scroll to the bottom of the page
* Expand the "Maintenance settings" section by clicking on it
* Enable debug mode for everyone
* Leave display of notices switched off
* Save the new settings by clicking on "Save new configuration"
* Go to the "groups" panel
* Set the upload form configuration for all groups to "Single file uploads only" (set File upload boxes to "1", URI upload boxes to "0" and No. of boxes to "fixed")
* Save your new settings
Then try to upload (using http uploads, even if you experienced troubles using another upload method) - you should get a more detailed error message that tells you what exactly goes wrong with your uploads. If the error message doesn't mean anything to you, search the support board for the error message you get.
When asking for support on the coppermine forum, post a link to your site and a test user account (the test user mustn't be in the admin group!) with upload privileges, with the above mentioned settings in place - this way, supporters can see the error messages as well. Do not post debug_output unless requested.
When people have issues with uploading and decide to post their question on the Coppermine support board, they usually are told to read this upload troubleshooting section. Many of them fail to do so properly, which results in frustration both for users as well as supporters. To make this absolutely clear: the above mentioned steps are absolutely mandatory, no matter what skill level you have, no matter what upload method you have troubles with. Failing to do exactly as suggested will result in your request for help being ignored. Yes, this applies to you. We mean it!
I wasn't trying to be sarcastic.
I'm doing my best trying to sound sarcastic in this posting. It is very frustrating for supporters if people don't do as suggested. What else can you do expect becoming a bit sarcastic

Funny how a lot of people (myself included) seem to be having this "add file/no thumbnail" error.
You haven't reported an error message that says that. You only have reported a "403 forbidden" error.
Now let's start over with this thread: do
exactly as suggested in the upload troubleshooting section of the docs. If you do that in your next posting, we will be able to continue this thread and give you actual support. If you don't (and just post a useless rant instead), I'll lock this thread and ban your user account, as I know better ways to waste my time than repeating the same mantra over and over.
Joachim