After enabling debug mode, if your server settings allow it, ProjectSend will enable the output of warnings and errors.
While php will most likely log them into an error.log file, there are 2 places inside the app where you can look for visible errors:
Any regular page will display warnings and fatal errors in the place where they happened. In most cases, parts of the page will be rendered and the error will appear after that. If somehow it's inside an HTML element it might not be visible in the page directly, but you will be able to find it by inspecting the page's code (a regular browser's function).
If your server has XDEBUG for php installed and enabled, the output will be easier to read and have even more details.
Warnings, notices and fatal errors can appear in place when debug is properly enabled and php's configuration allows it.
Ajax calls (loading the dashboard widgets, creating a zip file, downloading the actions log, etc) can be found by opening the browser's console and checking the Network tab.
Check the following image to see the relevant places: Inside the Network tab, XHR (Ajax) calls were filtered, then a request was selected, inside that call, the response sub-tab was selected and finally the output appeared on the bottom box. In this case, the call did not return any errors, but this is where they can be found if output is enabled.
Browser's console showing the Response tab inside an ajax request, where errors might appear if the request fails.