WebFMC dev here. This is a single instance where we call XPLMDebugString() from a background thread during early startup. No other calls to XP API are made from threads ever. If this log access was ever an issue, I'm sure we would have known already after ~3 years of the plugin existence (plus, it would crash very early, even before aircraft is loaded). Anyway, since Laminar's guidance on threading-safety is pretty clear, I'll eliminate this case in the next update of WebFMC, to be on the safe side.
Anyway, I'm glad the issue has apparently been resolved. And the cockpit with 2 FMC displays looks absolutely gorgeous