Tried contacting the developer and they were kind enough to respond.
The issue was, when the donate button was clicked, I was sent to the right page which is “http://nabtron.com/donation_can_ipn/start_donation” however it stopped there instead of redirecting to the paypal page.
I tried reinstalling the donation can plugin, disabling the cache plugin (w3 total cache) but still no luck.
But One thing that did work was disabling all plugins, testing the plugin to be working fine and then re-enabling them one by one.
i was expecting to find the plugin which is conflicting with donation can plugin but instead they all work fine now.
I am still suspecting w3total cache for this episode but anyway.
So the tip is, when you have issues with plugins having conflict with each other, simply disable them and enable one by one and see if this solves the issue.
Another thing to try is to revert to the default theme for a while to confirm if the theme is the issue or not.
Make sure all your plugins, themes and the wordpress core is always up to date.