After some digging I found plenty of unconnected issues, but finally managed to solve them and wanted to share them all in one place so nobody else waste that much time if hits the same issue (or some of them).
First you want to access is the Reporting Services Error Catalog, depending on your installation directory, it will be something like this:
?:\Program Files\Microsoft SQL Server Reporting Services\SSRS\LogFiles
As there will be multiple files I recommend you to stop the SSRS service (PS as admin):
Get-Service -Name SQLServerReportingServices | Stop-Service
Then remove all of the files there (still in the same PS session):
Remove-Item -Path ?:\Program Files\Microsoft SQL Server Reporting Services\SSRS\LogFiles -Force
This might fail for the ReportingServicesWMI_YYYY_MM_DD_HH_MM_SS.log file as it is in use, but there is no need to stop the WmiPrvSE.exe here, you can leave it as it is.
One you have your log space cleared you can start the SSRS service again:
Get-Service -Name SQLServerReportingServices | Start-Service
And access the Web Portal again. Go as deep as you need to access the error again and stop the SSRS service again. At this point you should be able to find your error message in one of the error log files.
As your can differ I will just list all of the fixes I had to apply:
Give Full Control to the service user I used to run reporting services over the:
?:\Program Files\Microsoft SQL Server Reporting Services\SSRS\ReportServer\RSTempFiles
Give SSRS MSSQL Login permission to View any database
USE [master]
GO
GRANT VIEW ANY DATABASE TO [DOMAIN\SSRS_MSSQL_Login] AS [sa]
Allow Log on as a batch job to SSRS service user:
RUN --> secpol.msc
Security Settings --> Local Policies --> User Rights Assignment --> Log on as a batch job
Add User or Group
Reference Material
