Recently dealt with an issue which is related to Forms Authentication on SQL server reporting services.
The root cause of the issue was some missing pieces in the implementation of Forms authentication on the reporting services.
Below steps were followed to resolve the issue,

Open the web.config  file from below location where the reporting server is installed.

Find authentication node and change the loginurl value as per your required url.
Add below Section after authentication section,
<authorization>
<deny users="?"/>
</authorization>

Save and close the file.
Repeat the same steps to Web.config in Report Manger folder.

Restart the Reporting Services.

Thank you.