I recent ran across a case where a customer had recently migrated to Exchange 2013. As one of the final steps in the migration, they migrated their public folders to modern public folders on Exchange 2013. After the public folder migration was complete, the users were unable to delete or modify pre-existing public folder items even though they appeared to have the correct permissions. Users tried modifying these items in Outlook 2010 and 2013, and in cached mode and in on-line mode all with the same results. After a bit of testing, it was discovered that this issue only occurred for public folder items that were migrated, and not for newly created items. Checking public folder root permissions showed no issues.
One thing that did help was changing the permissions for the user from Author (Edit and Delete Own) to Editor (Edit and Delete All) on the public folders, however this granted users more permissions than they should have so it was not really a workable solution.
As a further troubleshooting step, we opened some of the effected items with MFCMapi to ensure that PR_CREATOR_ENTRYID property was valid, thinking this might be similar to an Exchange 2010 migration bug where that field was cleared. Alas, this got us no closer to a solution as everything was as it should be there.
After a more aggravation and dead ends, we came across the following proposed solution
Export PF Calendar to .pst
Run Scanpst.exe on it and fix any issues
Import PF Calendar back to PF
Apparently the cause is corrupted permissions and this resolves the issue. Another problem solved thanks to the brilliant minds on the Exchange ranger distribution list.