I have a third party developer that is creating Lead records in Salesforce from a mobile app.  This has been working for months, so that I stopped testing it months ago.  Now, as part of end-to-end testing, I have come back to this functionality - and it's not working.  I have no idea why.  As far as I know, niether I nor the third party has changed code that should affect this.  Further, my logs don't show any errors,altough their own logs do show an error.

Basically, looking at my logs, what happens is that the mobile app submits the Lead to Salesforce, Salesforce gets the Lead request, and then stops.  Complete log record:

Request Type;  API
Status:  Success

The complete log record:

42.0 APEX_CODE,DEBUG;APEX_PROFILING,INFO;CALLOUT,FINEST;DB,FINEST;NBA,INFO;SYSTEM,DEBUG;VALIDATION,INFO;VISUALFORCE,INFO;WAVE,INFO;WORKFLOW,FINER 11:28:06.0 (647343)|USER_INFO|[EXTERNAL]|<ACTUAL LOG HAS SANDBOX USER INFO HERE>|Pacific Standard Time|GMT-07:00 11:28:06.0 (754319)|EXECUTION_STARTED 11:28:06.0 (760660)|CODE_UNIT_STARTED|[EXTERNAL]|Flow:Lead 11:28:06.0 (1172495)|CODE_UNIT_FINISHED|Flow:Lead 11:28:06.0 (1186927)|EXECUTION_FINISHED

That's it.  That's the complete log.  The third party developer says that they had the Lead creation fail once last night, and then the rest were successful.  Mine aren't succeeding at all.  The Salesforce logs for their successful attempts have exactly the same messages as above, but then continue on showing it firing my triggers and additional code I've got set up.  When I submit, it doesn't seem to fire my code at all.

No Lead is created when it fails.

For their failure, in their own logs, they got the error:

"Error: Invalid record ID. Specify valid record ID value"

Since this isn't hitting my own code at all, I don't have any system.debug information.

Any idea what might be going on?