Why can I not upload files to a website on my Macbook Pro.

Neomegaresin wrote:

I know it's not the site because I can access it from another computer.

So "Zenfolio" is a CMS website builder— and you are using their interface to upload content and you are having issue on this one isolated machine— have you ask their IT depart for support on the issue?

To trouble shoot further you can:

 —Try a SafeBoot https://support.apple.com/en-us/HT201262

  Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache,  etc.  Login and test. Reboot  as normal and test as caches get rebuilt.

In Safe mode third party system modifications and system accelerations are disabled, it removes malware, etc hampering smooth operation, however a  reboot will put it back to normal mode.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode. 

 —Test issue in another user (or guest user) account  https://support.apple.com/en-us/HT204443

  This will tell you if it a universal issue or isolated to your user/admin account. 

If you narrow it down to your user account you can drill down deeper—To get a good look at your System config. and reveal conflicts or issues, you can download/run this trusted utility https://etrecheck.com  

If you need help interpreting the report you can post it here in its entirety in the "Additional Text" box in the editing toolbar below, in your reply.

Try reinstalling this in Safe Mode, but not using Firefox...

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Safe Boot, (holding Shift key down at startup).

Seems to fix about 90% of the problems.

Neomegaresin wrote:

It seems that since the last update of Mojave, I cannot upload file, photos etc to any websites other than Facebook or Whatsapp. The upload button just won't work on any site.

How can I rectify this. And, no, I'm not going to upgrade to a higher OS as I have too many programs that still run on 32bit before you ask, so I'm marooned on Mojave it seems.

You would have to be more specific— give an example, share a link

"any websites other than Facebook or Whatsapp."

Try sharing an image here for example:

Good work, take care my friend. :)