As a quick check before disabling the firewall, what about if you log into the SSRS server and try uploading from the server itself? That SHOULD bypass the firewall.
Since you are using https for this now, is your certificate valid? What I mean is do you get any errors or warnings or any "Not secure" notices in the browser bar?
I also find it a bit odd that the SSRS logs don't contain any information about the issue... They should at the very least be telling you if it is a 5xx error or a 4xx error? What about if you load up the developer console in your browser and then try the upload. That too should tell you if it is a 5xx error or a 4xx error or something else.
Another thought - what about an incognito browser window? I am wondering if it could be an extension issue OR a cache issue. Also, do you have a password manager set up on your browser and if so, is it configured for your SSRS site? I know we've run into issues with password managers that are set to auto-fill and have had to get end users to disable them.
My very last thought is how large is the file you are uploading? I am wondering if you have a limit on the file size to be uploaded to the server.
I am thinking it is not firewall related only because you said other files upload no problem, but it also doesn't hurt to try disabling the firewall like Phil suggested temporarily to see if it helps.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.