We had the same issue and reserved set memory for SQL and left the rest as shared.
There also seems to be one possible solution on:
https://social.msdn.microsoft.com/Forums/sqlserver/en-US/b80d7f29-b271-4845-b7be-df5a9b591ed2/ssrs-2008-connection-failure-timeout-or-low-disk-condition-within-the-database?forum=sqlreportingservices
Quoted as follows:
"
Rich,...