Well, not so much fixed, because the account is needed to authenticate to some web service data sources. But the behaviour suggests to me that SSRS is getting it's Oracle...

Another twist is disabling the execution account, or rather removing it from the configuration, and reports run fine.

More info, this is SSRS 2016.

Surprised you didn't mention it, but I use an extended events session that I leave running to capture the login and source (among a few others) for a lock_acquired event....

to minimize service disruption you can restore with no recovery on your new cluster your last full backup from the old. then during a maintenance window just run a differential...

Another option for data file space consumption monitoring, and what I use, is policy based management.

I have two conditions that test free space. One that tests for >= 10% free...