While attempting to preview a report developed by another user (after getting it out of Visual Source Safe), I was confronted by the following error:
An error occurred during local report processing. Access Denied. (Exception from HRESULT: 0x80030005 (STG_E_ACCESSDENIED))
I created another report exactly the same and it seemed to work fine - so there was some other kind of caching issue causing this error.
After investigating, I noticed that the REPORTNAME.rdl.data file (which is a binary file and is how SSRS reports cache their preview information) - was read-only. SSRS was trying to write to the file and consequently getting the access denied errors. This was because the rdl.data file had erroneously been put into source control.
The fix was to remove it from source control (as it should be generated as each user previews the report), and mark the local file as read only.
Of course it would be much nicer (and simpler to troubleshoot the issue) if this particular "Access Denied" error message was more descriptive and actually told me *which* file it was trying to access.