Recently we faced the below error when running a SSRS report on AX2012 Feature pack CU4 environment:
This was LIVE environment with lots of users logged in, so we wanted to avoid any downtime, and the following helped us to fix this issue without any downtime:
1. Clear *.auc files
2. Restart SSRS server
3. Redeploy the report
There is a blog from MS support team on the same topic here which suggest that only redeployment of the report can fix this issue but in our case we were also required to restart SSRS services.