I suppose the ping is destined to the peer router's interface.
Do you have access to the peer device ?
You can either check it (or ask peer administrator to check it) for errors.
You are not seeing any on your side,
but maybe there are errors only in the transmit path
from you towards the far end equipment, and not the other way round.
In the meantime, put some pressure on the circuit's provider.
They changed something, you did not.
Statistically it's most probably their fault.
Do you know why they switched from main to protection ?
Do they already have some problems in their network ?
Do they intend to switch back to the main path, and when ?
