The error message reads:
Failed to run permission check: Unrecognized target entity table (civicrm_note)
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As Francesc pointed out, the symptoms sounds very similar to a contemporaneous question -- Why unexpected attached files are loaded in all mailings by default?
That was patched with https://github.com/civicrm/civicrm-core/pull/6644 and released as part of 4.6.9.