@tdsoft Thank you for your reply. I'm the hosting provider. It's my dedicated server. AFAIK, Zip is enabled in PHP - that's why I showed the output from phpinfo(). Is there something I'm missing?
I have tried uninstalling php_zip and reinstalling it. When it is uninstalled, the modules page fails silently when trying to install. Once reinstalled the page fails as above.
Interestingly, if I try to add a free module (I just tried "Advertising Block") it works fine. It's just for the paid for modules that it doesn't work. Again, I can download the module to my PC and then install directly using the "Upload a module" button. The module installs perfectly (if slowly!).