Please check the error log. The log file path for 2017 and above versions is: C:\Program Files\Microsoft SQL Server Reporting Services\SSRS\LogFiles
Error log is very helpful for resolving issue.
Modify the following information in the configuration file (rsreportserver.config) located at: C:\Program Files\Microsoft SQL Server Reporting Services\SSRS\ReportServer:
<Add Key="SecureConnectionLevel" Value="0"/>
Make sure the value is 0.