I'm getting this message also, exactly one of them.
###!!! [Child][RunMessage] Error: Channel closing: too late to send/recv, messages will be lost
This is output when I choose a profile and click Start Thunderbird at which point the Choose User Profile dialog closes and the app exits instead of opening the main window. This is version 91.0.1 on openSuse, on a new install with my Thunderbird data having been freshly migrated from another machine running Thunderbird 68.8.1.