Previous iCloud stored messages won't load in Messages on Big Sur

On my M1 iMac running Big Sur (11.6) when viewing any message thread in Messages, scrolling up to view previous (non-buffered) messages triggers Messages to display an animated loading wheel/dial that indicates an attempt to download older messages from iCloud. However, the wheel/dial spins forever without successfully downloading older messages. Retrieval of older messages from iCloud with Messages on iOS 14 and 15 on an iPhone XS works as expected, so the problem does not appear to be with iCloud.

The following actions were taken on my M1 iMac to fix the problem - none have worked:

1) Turned off Enable Messages in iCloud in Messages Preferences and then Signed Out of Messages, restarted the system, Signed In to Messages and turned on Enable Messages in iCloud.

2) Tried #1 again, but removed the Messages folder from my user Library folder before signing in and reactivating messages in iCloud.

3) Tried #2 again, but removed com.apple.Messages.plist from Library->Preferences in addition to (and at the same time as )the Library->Messages folder.

Messages preferences are set as follows:

Other suggestions for how to fix this problem?