Hi,
Perhaps try running reverification.
Go to policies management, find the replication policy that fails and select option (in advanced settings or parameters) "Reverify All" do it before the weekend as this may use all you've got of WAN and PD (but 100 Mbit/s seems quite cool and more than sufficient for 300 GB replication - 5 Mbit/s would do for PD).
Another option is to create a "brand new" data selection for this and after manual backup completes: replicating it.Then disabling the old data selection's replication as it is not going to change anymore.
Good luck!
MichaĆ