Posting this in case it's helpful to others.
I have a paginated report in PBIRS, and I have a data-driven subscription on that report. When the subscription ran, I got "Done: 5 processed of 5 total; 5 errors."
I could find details on these 5 errors. I looked in C:\Program Files\Microsoft Power BI Report Server\PBIRS\LogFiles and sorted by Date modified descending. I looked in the RSPowerBI_*, RSPortal_*, RSHostingService_*, and RSOffice_* files because those were up top. I eventually found my errors in ReportingServicesService_* which had a date modified *before* the subscription ran, but the log had the errors I was looking for. I suppose the log file had not been flushed until I attempted to open it, thus the date modified had not changed.
C:\Program Files\Microsoft Power BI Report Server\PBIRS\LogFiles\ReportingServicesService_[date].txt is the file that contains your subscription errors.
