The following error is observed in the Mail Synchronization Agent log (MSA), which is preventing the MSA from marking the user as being completed.
FolderFilter::ExcludeFoldersContent Error 457 This key is already associated with an element of this collection
MailKernel::IsReadyToComplete Informational 2085 Synchronization status: Errors found: 1, allowed: 0.
When using the SkipFoldersContentMask parameter as described in the following article, there is a known issue (TF 209053) that will be encountered as a result:
Due to this error being encountered, this will result in the MSA not marking the user as "Completed" and subsequently results in the Calendar Synchronization Agent not reversing synchronization direction.
Further information along with other causes for one-way Calendar synchronization not switching direction can be found here, which also includes a log example of the MSA marking a user as "Complete"
To resolve this issue, the "MaxErrors" parameter must be updated within the associated MSA Config.mdb file - please use the following steps: