{"ClassName":"System.UriFormatException","Message":"Invalid URI: The URI is empty.","Data":{"OperationId":"0ac97f7603ad4ac8940f63a8838a8fc8","$$$Reported":"OperationLogger"},"InnerException":null,"HelpURL":null,"StackTraceString":"   at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind, UriCreationOptions& creationOptions)\n   at System.Uri..ctor(String uriString, UriKind uriKind)\n   at Laserfiche.Blob.FileStoreDirectEngine.ResolveBlobId(String id)\n   at Laserfiche.WebRequests.Messa...

Thanks, that quickly led me to where the error is coming from:

Laserfiche.Blob.FileStoreDirectEngine.ResolveBlobId(String id)

That function takes in a blob storage object ID and appears to generate a URI based on it. For a pseudocode example, something like this:

Laserfiche.Blob.FileStoreDirectEngine.ResolveBlobId("abc123") == https://s3.amazonaws.com/examplebucket/abc123 

What's plausibly happening is that for whatever reason, either a null or invalid blob object ID is getting passed to the function, causing the URI generation to fail, causing whatever validates that generated URI to throw the 

"Invalid URI: The URI is empty."

error you're seeing. The error comes from a low level operating system library exception "System.UriFormatException", which is why I couldn't find it in our code. 

So long story short, it looks like the error isn't about the URI you're telling the Web Request Rule to call, but rather a different URI that Cloud generates for some request handling purpose. Perhaps that ResolveBlobId URI is where Cloud temporarily stores the Web Request response. Just a guess.

Anyway, I don't know offhand how or why your Cloud Web Request Rule ends up calling this ResolveBlobId function or why the it's failing to generate a valid URI. Maybe a bug or unhandled edge case, maybe a configuration issue, maybe something else.

Either way, definitely a support case. They'll have the backend visibility necessary to troubleshoot this further (I do not). You should link this Answers post in the case so Support can pick up where my investigation left off. Tell them I said to look at "Business Rules Task #508796", which while about a different issue, contains a detailed log file that happens to include this same specific error / stack trace pattern.