After upgrading to 4.1, we see "fbl: Header must be in JSON format since Momo 4.1. error: unexpected character" in paniclog.ec
If you recently upgraded to Momentum 4.1 or later, you may notice an influx of the following message in your /var/log/ecelerity/paniclog.ec:
fbl: Header must be in JSON format since Momo 4.1. error: unexpected character
This log entry will be written each time your new >4.1 system receives an incoming FBL message that contains the legacy X-MSFBL header. Please note that this error does not mean the message was not properly processed. The FBL module is backwards compatible and will still read and process legacy X-MSFBL headers. You can confirm this is the case using the fbllog.ec and rejectlog.ec.
Unfortunately, if for any case you decide to downgrade from 4.1+ to something earlier, the earlier versions will not be able to read the JSON format X-MSFBL header.