by Eric York.
We received a similar error upon upgrade to 2.6. For us, it happened when annotated PDFs were batch uploaded and it blocked access to the single grade page, so the bad files could not be removed. We resolved it by disabling AnnotatePDF across the site, until we can upgrade to a fixed version. Here is a relevant forum post: https://moodle.org/mod/forum/discuss.php?d=243971 and here is perhaps the related tracker issue: MDL-43593. Only a site administrator can disable AnnotatePDF site-wide.