Ubuntu 21.10 Thunderbird - IPDL protocol error: Handler returned error code!

Thunderbird 91.3.1

If I start Thunderbird from the terminal, and then select any email message to read, I see the following error...

IPDL protocol error: Handler returned error code!

###!!! [Parent][DispatchAsyncMessage] Error: PClientManager::Msg_ForgetFutureClientSource Processing error: message was deserialized, but the handler returned false (indicating failure)

Google doesn't give any real clue about what an "IPDL protocol error" is.

To troubleshoot, I logged into a separate Ubuntu account where Thunderbird had never been used. I started Thunderbird from the terminal and created a mail account using one of my Google gmail accounts. Read any message, and got the same problem. So it's not an account-specific problem.

Partial log from terminal...

~$ thunderbird
[calBackendLoader] Using Thunderbird's libical backend
[LDAPModuleLoader] Using LDAPDirectory.jsm
[MsgSendModuleLoader] Using MessageSend.jsm
[SmtpModuleLoader] Using SmtpService.jsm
console.debug: WebExtensions: 
  Compact Headers loaded
1642775310275   tbsortfolders.folderPane    DEBUG   Init
1642775310275   tbsortfolders.folderPane    DEBUG   Add observer
console.debug: "Trying to load /usr/lib/thunderbird/libotr.so"
console.debug: "Trying to load libotr.so from system's standard library locations"
console.debug: "Trying to load libotr.so.5 from system's standard library locations"
console.debug: "Successfully loaded OTR library libotr.so.5 from system's standard library locations"
console.debug: "Successfully loaded OpenPGP library librnp.so version 0.15.2+git20210806.dd923a4e.MZLA from /usr/lib/thunderbird/librnp.so"
console.debug: "Found 0 public keys and 0 secret keys (0 protected, 0 unprotected)"
console.debug: "Successfully loaded optional OpenPGP library libgpgme.so.11 from system's standard library locations"
console.debug: "gpgme version: 1.14.0-unknown"

IPDL protocol error: Handler returned error code!

###!!! [Parent][DispatchAsyncMessage] Error: PClientManager::Msg_ExpectFutureClientSource Processing error: message was deserialized, but the handler returned false (indicating failure)

IPDL protocol error: Handler returned error code!

###!!! [Parent][DispatchAsyncMessage] Error: PClientManager::Msg_ForgetFutureClientSource Processing error: message was deserialized, but the handler returned false (indicating failure)

Any ideas what this error is about?