I just enabled debugging/diagnostics for Serilog and below is the generated exception:
2023-09-04T07:08:46.0982059Z Exception while emitting periodic batch from Serilog.Sinks.MongoDB.MongoDBSink: System.AggregateException: One or more errors occurred. —> MongoDB.Driver.MongoCommandException: Command insert failed: Unsupported OP_QUERY command: insert. The client driver may require an upgrade. For more details see https://dochub.mongodb.org/core/legacy-opcode-removal.
I guess this is due to version incompatibility between the MongoDB sink and the server. For now, I’ve asked IT team to install the server version which worked before.