Channel Pipeline describes the rather complex internal flow. I tried to find main path. It might be:
FileEnumerationProcess.cs → SourcePaths → MetadataPreProcess.cs → ProcessedFiles → FilePreFilterProcess.cs → AcceptedChangedFile → FileBlockProcessor.cs → StreamBlock → StreamBlockSplitter.cs → OutputBlocks → DataBlockProcessor.cs → BackendRequest
and your error message is from FilePreFilterProcess, so still pretty early before the reading of file.
I suppose you could try a log-file=<path> which by defalt would log the warning with a stack trace.
log-file-log-level=verbose will get individual source files, and maybe some further clues, although I guess we have at least a file name that’s upsetting it.
It will make a huge file, but you could try a log at profiling level, which will record things like below:
2024-08-06 05:25:51 -04 - [Profiling-Timer.Begin-Duplicati.Library.Main.Database.ExtensionMethods-ExecuteNonQuery]: Starting - ExecuteNonQuery: INSERT INTO "FilesetEntry" ("FilesetID", "FileID", "Lastmodified") VALUES (757, 12839, 636811898376551414)
2024-08-06 05:25:51 -04 - [Profiling-Timer.Finished-Duplicati.Library.Main.Database.ExtensionMethods-ExecuteNonQuery]: ExecuteNonQuery: INSERT INTO "FilesetEntry" ("FilesetID", "FileID", "Lastmodified") VALUES (757, 12839, 636811898376551414) took 0:00:00:00.000
Looking in the database might be necessary. Creating a bug report is sometimes enough, but the actual filenames are not in there (for privacy). You looking under direction might also be possible.
Help from a developer who knows the code well would help, but they’re scarce, and usually busy.