You may want to start with looking at the odoo log file

Nothing in the odoo log file.

I found that it's an nginx timeout error, so I increased the imeout in nginx config file. Now I get the report but I have to wait 5 minutes even if it's a very small file. Before it took 5 seconds to have the report, and now 5 minutes. I don't understand why.

not sure this will improve the time issue, anyway you can have a try, activate the developer mode and navigate to settings -> technical -> parameters -> system parameters , and check whether there is a record with key as report.url, if so check whether its value is same as the one you use for accessing the instance. Suppose if your site address is niyasraphy.odoo.com , make sure that the report.url key's value is the same. if there is no such a record with this key, create one.

I added the parameter report.url in the system parameters. It takes now a minute for the report to display, but the report is missing the CSS.