Error sequence: message altered/invalid passphrase, then missing files

I had a weird sequence of errors on a job that has been running (backing up to Dropbox) successfully for a long time. On 8/25 I got this error:

2019-08-25 06:27:38 -07 - [Retry-Duplicati.Library.Main.BackendManager-RetryGet]: Operation Get with file attempt 1 of 5
failed with message: Failed to decrypt data (invalid passphrase?): Message has been altered, do not trust content

which caused the backup to fail. Then the next day I got:

2019-08-26 04:35:15 -07 - [Warning-Duplicati.Library.Main.Operation.FilelistProcessor-MissingFile]: Missing file:
2019-08-26 04:35:15 -07 - [Error-Duplicati.Library.Main.Operation.FilelistProcessor-MissingRemoteFiles]: Found 1 files that are missing from the remote storage, please run repair
2019-08-26 04:35:15 -07 - [Error-Duplicati.Library.Main.Operation.BackupHandler-FatalError]: Fatal error
Duplicati.Library.Interface.UserInformationException: Found 1 files that are missing from the remote storage, please run repair

I get these “found X files that are missing” errors on jobs from time to time, but this is the only time I recall seeing an issue immediately before that. Not sure if the issues are related but thought I’d post it here in case it helps anyone better understand the missing files issues that some of us seem to have. There’s no reason I can think of why the first message would have happened; the job has been running for a long time, I haven’t moved installations, upgraded, or edited anything, and the passphrase hasn’t changed.

Was just checking the forum for unanswered posts and saw this one. Were you able to resolve this problem?

The only way I’ve ever been able to successfully stop the “missing files from remote storage” error is to delete all the remote files, wipe the database, and re-run the job as it if was a new backup. I’ve tried the process of removing the offending backup set but it always turns out that the error returns.

Fortunately it hasn’t happened in awhile now.