It turned out I didn’t have to do anything, as my scheduled backups on Jan 5 and 6 failed too:
I think some of my tabs reconnected on their own, and I did Dismiss the red popup notification.
Maybe I’ll wait a little longer in case there’s something else that can be gleaned from this case.
EDIT 1:
Stacks are different. Original looks like on the way out. Next two on the way in. Nothing in logs, including the Application event log.
EDIT 2:
Since scheduled backups failed reliably, I tried one manually with breakpoint at StartScope.
Looking further got stack below, and it's about to throw new ArgumentNullException("entry");
> Duplicati.Library.Logging.dll!Duplicati.Library.Logging.FunctionLogDestination.FunctionLogDestination(System.Action<Duplicati.Library.Logging.LogEntry> entry) (IL≈0x0016, Native=0x00007FFED807D280+0x83)
Duplicati.Library.Logging.dll!Duplicati.Library.Logging.Log.StartScope(System.Action<Duplicati.Library.Logging.LogEntry> log, System.Func<Duplicati.Library.Logging.LogEntry, bool> filter) (IL≈0x0000, Native=0x00007FFED807D160+0x4C)
Duplicati.Library.RestAPI.dll!Duplicati.Server.Runner.Run(Duplicati.Server.Runner.IRunnerData data, bool fromQueue) (IL≈0x016D, Native=0x00007FFED81398C0+0x78D)
Duplicati.Server.Implementation.dll!Duplicati.Server.Program.<>c.<SetWorkerThread>b__62_0(Duplicati.Server.Runner.IRunnerData x) (IL=epilog, Native=0x00007FFE66506300+0x21)
Duplicati.Library.Utility.dll!Duplicati.Library.Utility.WorkerThread<Duplicati.Server.Runner.IRunnerData>.Runner() (IL≈0x0191, Native=0x00007FFE664F24D0+0x503)
System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) (IL≈0x0040, Native=0x00007FFEC1246420+0x7D)
[Native to Managed Transition]